The forecast says 28°C. You've got a run on the plan, and the morning's argument has already started: push through, or skip it. Both options sound wrong in different ways.

Heat isn't a willpower problem. It's a physiology one, and once you know what your body is actually doing in the heat, the right call gets a lot easier to see. Most of the time the answer is yes, you can run. You just don't run the same run.

Here's what changes when it gets hot, and how to handle a hot spell so you come out of it fitter rather than fried.

Why heat changes the run

When it's warm, your body is doing two jobs at once. It's powering your muscles, and it's shedding heat to keep your core temperature where it needs to be. To cool you down, it sends a chunk of your blood supply to your skin, where it can dump warmth. That leaves less blood circulating to the legs that actually want it. To make up for the shortfall, your heart rate climbs. The work your body is doing on the run goes up while the pace stays the same.

That's why the easy pace that felt comfortable in April starts to feel like effort in the first proper heat of the year. It isn't that you've gone backwards. It's that the run is genuinely harder, and your body is correct to find it so. The cooling system is doing its job.

There's also an individual layer to this. Some people tolerate heat better than others, and the difference is real, not character. Body size, sweat rate, fitness level, where you grew up, and what your training summers have looked like all feed into it. If you've found yourself struggling in heat that other runners in your group seemed fine in, that isn't a flaw to push through. It's information about your body.

The first hot days are the hardest

Here's the part nobody tells you. The first hot days of a British summer are the hardest days of the whole year to run in. Not because the weather is at its worst, but because nobody is acclimatised yet.

When you spend repeated days running in heat, your body adapts. Your blood plasma volume increases, you start sweating sooner and more efficiently, your heart rate at a given effort comes down, and the same run starts to feel manageable. The adaptation takes roughly ten days to two weeks of regular exposure, with the biggest jumps in the first week.

This is good news. The first hot run of the year is, by some distance, the worst hot run of the year. Give your body a couple of weeks of warm weather and most of what felt unrunnable in week one becomes simply hot in week three. If your first run of a hot spell felt awful, it isn't a verdict on your summer. It's the start of an adaptation that takes a fortnight.

Adjusting the run

Once you accept that the run needs to change shape, the rest is logistics.

Move it to the edges of the day

Before nine in the morning is best, and anywhere in the evening is fine. The window worth avoiding is roughly eleven to three, when the sun is doing the most work. Even a 6am start can be several degrees cooler than midday, which is a lot of free relief for no extra effort.

Run by effort, not pace

Decide how hard the run should feel, and let your pace be whatever that effort produces. Easy in a heatwave is a lot slower than easy in April, and that's correct, not failure. If you train by heart rate, trust it: the same heart rate will give you a slower pace than usual, and forcing your usual pace on a hot day buys you no fitness and a much worse afternoon.

Pick a route with shade and water

Tree-lined paths, parks, and riverside trails are all cooler than open road. If you can loop past a tap, a shop, or a public fountain, better still. Carry water if you'll be out for more than thirty or forty minutes, and don't overdress: light colours, loose fit, things that breathe, and a cap if you're going to be in the sun.

Hydrate sensibly

Drink to thirst rather than forcing volume. If you're out for more than an hour, or sweating heavily, add a bit of sodium, either an electrolyte tablet during the run or salty food beforehand. For shorter runs in the heat, water is fine, and over-drinking can do more harm than good.

The stop signs

This is the part of the article worth reading twice. Heat illness is real, it can come on faster than people expect, and "I'll push through" is not a strategy worth using here.

Stop the run if any of these show up:

  • Dizziness or feeling faint.
  • A pounding headache that gets worse, not better.
  • Nausea, especially if it arrives suddenly.
  • Confusion or trouble thinking clearly.
  • Chills, goosebumps, or feeling cold in the heat.
  • Suddenly not sweating when you should be.

If any of those land, get to shade, sit or lie down, cool your skin with water if you can, and drink something. If symptoms don't ease within a few minutes, or get worse, that's an emergency. Heat stroke is a medical emergency, people die from it, and it is not something to ride out at home.

A swapped run, or a stopped run, is not a failed run. It's the right call.

Heat is training, not setback

Here's the reframe worth holding on to. The hot weeks aren't lost training. They're a different kind of training, and the adaptations stick around. Your blood plasma stays higher, your sweat is more efficient, your cardiovascular system is more practised. Some athletes train deliberately in heat to bank exactly these effects. You get them for free, just by running through a British summer.

I used to see a slow, ugly pace on a hot day as proof I'd gone backwards. What I learnt is that the pace was meaningless, and the adaptation was the point. The runner you are at the end of a hot summer is fitter than the runner you were going in, even if the watch never shows it. The pace comes back when the weather cools, and the heart you've quietly trained is still there.

A hot run is a different sport with the same kit. Move it to the cool of the day, run by effort, drink sensibly, watch for the stop signs, and trust that the first week is the worst week. By the time the second hot spell rolls round, the body that struggled through this one will be quietly ready. The runners who handle a heatwave well aren't the toughest ones. They're the ones who adjust.

This article is general information, not medical advice. If you have a medical condition affected by heat, or you become unwell during or after a run in hot weather, please speak to a GP or seek medical help.