Interval training after 40: how to get the intensity right

You have been running easy for a few months and it is going well. Then you read that you need intervals for anything to happen, and the following Tuesday you run four laps of the football pitch as fast as you can. The third lap hurts. The fourth becomes a negotiation.

On Wednesday your Achilles makes itself known. By Thursday you have decided that intervals are not for you.

The intervals were not the problem. The intensity was. Almost everything that goes wrong with speed work after 40 goes wrong in the first work interval, before the session has even become a session.

What an interval session actually is

An interval session is not "running fast sometimes". It is a structure: a defined work period, a defined recovery, repeated a defined number of times.

The point of that structure is that it lets you accumulate more time at high intensity than you could ever manage in one continuous effort. Four times four minutes is sixteen minutes at an intensity you could not have held for sixteen minutes straight. The recoveries are what make that total possible.

Which is why the recovery between intervals is part of the session rather than an interruption to it. Cutting it short to "work harder" does not make the session harder in any useful way. It just makes the next work interval worse.

Why intervals raise fitness faster than easy running

The clearest comparison comes from a Norwegian study that had four groups perform the same total work distributed in different ways (Helgerud et al., Medicine & Science in Sports & Exercise, 2007). Forty healthy, moderately trained men were assigned to long slow distance running at 70 per cent of maximum heart rate, lactate threshold running at 85 per cent, fifteen seconds hard alternating with fifteen seconds easy, or four repetitions of four minutes at 90 to 95 per cent of maximum heart rate.

After eight weeks the two interval groups had raised their maximal oxygen uptake by 5.5 and 7.2 per cent respectively. The two groups who ran at a steady pace barely improved theirs at all, despite having done the same amount of work.

One important exception: running economy, meaning how much oxygen you burn at a given pace, improved in all four groups. Steady running did do something. Just not to maximal oxygen uptake.

The mechanism sat in the heart. Stroke volume, meaning the amount of blood the heart pumps per beat, rose by roughly ten per cent. That increase appeared only in the interval groups.

Read those figures with the right caveats. Forty participants is not many, all of them were men, and all of them were already moderately trained. The study tells you something about what high intensity does to a heart. It tells you nothing about what is wisest for a 48-year-old who has been sedentary for a decade.

The honest limit: what intervals are not

Speed work is often marketed as something close to medicine. That claim has actually been tested, in the largest randomised long-term trial of exercise intensity and mortality in older adults.

Generation 100 (Stensvold et al., BMJ, 2020) followed 1,567 people aged 70 to 77 in Trondheim for five years. They were randomised to two weekly sessions of high-intensity intervals at about 90 per cent of peak heart rate (400 people), two weekly sessions of steady moderate training at about 70 per cent (387 people), or simply to follow the national physical activity guidelines (780 people).

The primary outcome was all-cause mortality. After five years, 4.6 per cent of participants had died. In the control group the figure was 4.7 per cent. Across the two training groups combined it was 4.5 per cent.

In other words, no difference. Compared against each other, the interval group sat 2.9 percentage points below the moderate group. The hazard ratio for that comparison was 0.51, with a 95 per cent confidence interval running from 0.25 to 1.02. Because that interval includes 1, the difference is not statistically significant.

That is also why this page is not trying to talk you into anything. If you manage three easy runs a week you already have most of the benefit. The interval session is the next step, not the entry ticket.

How hard is hard? Setting the intensity

This is the heart of the page. The word "hard" means different things to different people, and many of those who get injured doing speed work do so because their "hard" was in practice "very nearly flat out".

Breathing is the most reliable tool you have

The talk test works just as well going up in intensity as it does going down. You only need to know what to listen for.

A work interval belongs at "hard", meaning a few words at a time. If no words come at all, you ran too fast. That sounds simple because it is, though most people need a couple of attempts before the feel settles.

Heart rate lags, especially in short intervals

Heart rate is slower to respond than effort is. Start a three-minute interval and it takes roughly one to two minutes before your heart reaches the zone you are aiming for. Which means the display reads too low for the first half of every interval.

The usual consequence is that people run too fast chasing a number that would have caught up on its own. Judge the interval by how it breathes instead. Look at heart rate only towards the end of it.

The shorter the interval, the less useful heart rate becomes. In a fifteen-second effort it barely reacts at all. There, feel is all you have.

How the zones are calculated, why the age formula has such a wide spread, and what beta blockers do to the whole calculation are covered in the guide to zone 2.

The best self-check: the last interval

If you can run the final work interval at roughly the same pace as the first, you got it right. If the last two were clearly slower, you started too hard.

This beats any heart rate monitor, because it measures the session against itself rather than against a formula. It also accounts for how you happen to feel that day: after a bad night the correct pace is slower, and the last interval will tell you so.

The session types and what each is for

Interval sessions are not interchangeable. What separates them is less about pace than about how the load is distributed across your body.

Notice that hill repeats carry the lowest impact load even though the heart rate goes high. That is precisely their value. Running uphill forces you to slow down, which reduces the forces going through each footstrike, while the heart works every bit as hard. You get the intensity without the mechanical punishment.

Which is why this guide starts on a hill. For a comeback body whose tendons have not yet caught up with its fitness, it is the cheapest route to high intensity there is.

Your first interval session

It should feel almost too easy when you finish. That is deliberate.

• 10 minutes of easy warm-up. Genuinely easy, full sentences. Never skip this before a hard session.
• 6 × 30 seconds uphill. Pick an even, moderate slope. Run it under control rather than flat out. A few words should still come.
• Jog or walk back down and start the next one once your breathing has settled, often after about 90 seconds.
• 10 minutes of easy cool-down.

Around 30 minutes in total, of which three are actually hard work. That sounds like very little. It is also exactly why it works as a first session.

Eight weeks, one staircase

The table below rests on a single rule: add repetitions before you add speed.

Week 4 is deliberately lighter. Stepping back after three build weeks is not lost time. It is what makes weeks 5 to 8 possible.

The jump from week 3 to week 5 looks large in the right-hand column, from five minutes to eight. It is not, because the character of the session changes at the same time. Second for second, thirty seconds uphill is a considerably harder work period than two minutes on the flat. As the work period gets longer, the effort per second has to come down. Total hard time is therefore not comparable between the two halves of the table.

Week 8 lands you on four times four minutes, the format from the Norwegian study above. By then it has taken you two months to get there instead of one Tuesday evening.

What happens to your intervals before a race

The ladder above only goes up. If you have a race booked, the last two weeks work differently, and that difference is the opposite of what most people do.

The rule for a taper is that volume comes down while pace stays where it is. For your interval session that does not mean cutting it. It means doing fewer work periods at the same pace as the week before. If you are on four times four minutes, run two or three instead, at the same effort in each one. Keeping the number and slowing down is the worst trade of the lot: you end up more tired without holding on to the speed the taper exists to protect.

A taper does not make you faster. It makes you rested, which is what decides how race day feels. The full week-by-week version is in the guide on how to run a half marathon.

One hard session a week, not two

This rule does more for staying uninjured than anything else on this page.

A study on recreational club runners looked at how long recovery actually takes (Frontiers in Sports and Active Living, 2022). Twenty runners, ten women and ten men, completed an interval session of six times 800 metres. The researchers then measured their running gait and muscle function a day later.

Most had recovered within 24 hours. A minority had not, and those runners who still showed signs of fatigue after a full day were judged to be at increased risk of a running-related injury.

The conclusion is not that you need to measure anything. It is that the line between enough recovery and not enough sits roughly there, that it differs between people, and that whoever guesses wrong pays for it in tendons. Two days between hard sessions is therefore a sensible margin for most people over 40.

If you run three times a week, that means one interval session and two easy runs. Nothing more. And remember the rule that governs the whole week: never increase distance and speed in the same week. If you are building distance at the same time, the whole plan for that is in the guide on how to run 10k.

Which day the interval session lands on does not have to be decided in advance. If you wake up clearly below your normal heart rate variability after a bad night, running that day easy and moving the hard session is usually the better call. How to read that signal is covered in the guide to HRV.

Six mistakes that ruin an interval session

• Starting the first interval too fast. By far the most common error. The pace feels right while you are fresh, and then there is nothing left for the final efforts.
• Cutting the recovery short. Start the next work period before your breathing has settled and the session becomes one long moderate slog instead of an interval session.
• Skipping the warm-up. Ten easy minutes before high intensity is not negotiable, least of all in cold weather.
• Two hard sessions back to back. See the section above. This is the mistake that most often ends in an angry Achilles.
• Letting every run land in the middle. When the easy runs turn moderate and the hard session is also moderate, the week costs a lot and returns little. Why that happens is covered in the guide to zone 2.
• Chasing the heart rate number. During short intervals it lags behind. Trust your breathing instead.

Who should be careful, and when to seek medical care

High intensity asks more of the heart than easy running does. For the great majority of healthy people that is not a problem, but two groups should check with a doctor before starting: anyone with known heart disease or uncontrolled high blood pressure, and anyone taking medication that affects heart rate. Beta blockers also mean that heart rate zones stop lining up with the effort.

• Call emergency services for chest pain or pressure that does not ease within a few minutes of rest, particularly alongside breathlessness, cold sweats, nausea or feeling faint.
• Contact a doctor if you get pressure or pain in the chest on exertion that eases when you rest, even if it only lasts a minute. It can radiate to an arm, the neck, the jaw or the back. New symptoms on exertion should always be assessed.
• Contact a doctor after fainting or feeling close to fainting during a session, or for heartbeats that feel irregular or race without exertion.
• Do not train hard with a fever, a sore throat or an infection on its way. Postpone the interval session. The easy run will usually wait just as well.

This is general health information, not a diagnosis or personal medical advice. If you are unsure, speak to a clinician.

Frequently asked questions

How often should you run intervals?

One session a week is enough for the great majority of people over 40. If you train three times a week, that means one interval session and two easy runs. Two hard sessions a week suits someone with several years of uninterrupted training behind them, and even then it rarely pays off for a comeback body.

How long should the intervals be?

Start at 30 seconds and build upwards over a couple of months. Work periods between two and four minutes are the most studied length for raising aerobic fitness. Shorter intervals give you high intensity with less accumulated fatigue, longer intervals train your ability to hold a faster pace. Both work.

Can I run intervals as a beginner?

Not as your first step. Build a few months of regular easy running first, until you can jog continuously for at least 20 to 30 minutes. That whole route is covered in the guide on starting to run after 40. Fitness responds in weeks while tendons and bone respond in months, so it is your base that decides when speed work becomes reasonable.

Should my heart rate reach zone 5 during intervals?

Not as a target. Aim for 90 to 95 per cent of maximum heart rate towards the end of each work interval. Heart rate briefly nudging into the top zone on the final repetitions is normal, but a session that sits there throughout was started too hard. Remember too that heart rate lags one to two minutes behind the effort.

How do I know if I ran too fast?

Compare the first work interval with the last. If they were roughly equally quick, you got it right. If you lost noticeable pace on the final two, you started too hard. Another sign is still feeling heavy-legged two days later.

Are hill repeats better than intervals on the flat?

They are gentler, which makes them a better first step. Running uphill forces you to slow down, so the load through each footstrike is lower while your heart rate still goes high. If you later want to train holding a faster pace, you will need to add intervals on the flat, because a hill does not train that.