In a heatwave, change how you run each session and leave the training plan alone. Steer by heart rate or effort instead of pace, and run in the coolest part of the day. Expect one to two weeks before your body adapts. Move or skip any hard session the heat makes unsafe, but keep the weekly structure as planned.
Run your usual easy 8K on a 29°C morning at the pace you held all spring, and the watch will call it easy while your heart rate calls it a threshold session. The pace looks fine. The heart rate sits well above where it normally does for that effort and doesn’t come back down, and the second half is slower than the first for no reason except the temperature. That gap between what the pace says and what the body is doing is the whole case for steering by effort, not pace, whenever it’s hot.
Why does heat slow you down?
Heat slows you down because your body has to divert blood to the skin to shed the heat running produces. That blood would otherwise be carrying oxygen to your legs. Core temperature climbs, perceived effort climbs with it, and the only lever left is to slow down. Marathon performance peaks in cool air. An analysis of about 1.8 million finishes in six big-city marathons put the best temperatures between 3.8°C and 9.9°C, depending on the runner. Above the optimum, speed drops and more runners drop out. Skoma’s marathon heat calculator puts rough numbers on it — about 1% slower near 15°C, 3% near 20°C, 6% near 25°C, and 10% or more by 30°C.
The exact percentage matters less than the principle: heat is a training stress in its own right, stacked on top of whatever session you planned. Ignore it and you aren’t running your plan in the heat — you’re running a harder, uncounted version of it.
Change: steer by effort, not by the watch
The single biggest adjustment is to stop steering by pace and steer by heart rate or perceived effort instead. On a hot day, run to a heart-rate zone and let the pace fall wherever it falls. Work out roughly where your zones sit with the heart rate zones calculator before the heat arrives, so on a hot morning you aren’t guessing at a number. Heart rate drifts up before pace visibly falls apart — by the time you notice the slowdown, you are already behind on the adjustment.
Change: when the run happens
The easiest adjustment of all: move runs to the coolest part of the day. In most places the difference between 6 a.m. and 2 p.m. in a heatwave is the difference between warm and genuinely dangerous, and that difference costs nothing. Long runs go as early as you can face; hard sessions go to whichever window is coolest, dawn or after sunset. A threshold session at midday in a heatwave isn’t dedication. It’s a worse session with more risk and no extra benefit.
Change: hydration starts the day before
By the time you feel thirsty on a hot run you are already behind, and mid-run drinking rarely closes the gap completely. So hydration for a hot session starts the evening before, not with the bottle you carry. Drink normally and consistently through the day before a hot long run, and don’t treat the run itself as the only chance to top up.
How long does heat acclimatisation take?
Heat acclimatisation takes one to two weeks of regular running in the heat. The first week of a heatwave usually feels the worst, and that isn’t bad luck. It’s a body that hasn’t adapted yet. Sweat rate, plasma volume and the point at which you start sweating all shift over that window. The sports-medicine consensus on training in the heat recommends repeated exercise-heat exposures over 1–2 weeks. Until they add up, every run feels disproportionately hard. Expect that window and don’t panic inside it: a session that feels broken in the first week of a heatwave can feel normal a fortnight later with nothing else about your fitness having changed. The mistake is reading an acclimatisation problem as a fitness problem.
Should you skip hard sessions in the heat?
Skip or move a hard session when the heat makes its target intensity unsafe. Not every session can be adapted. A VO₂-max interval set at 35°C in full sun isn’t a harder version of the workout — it is a different, riskier activity wearing the same name on the plan. If a session’s purpose depends on hitting a specific intensity and the heat makes that intensity unsafe, move it onto a treadmill or skip it. A missed interval session is a non-event; heat illness is not.
Know the warning signs and stop when they appear rather than when the session is done: dizziness, nausea, headache, confusion, chills or goosebumps in the heat, or skin that feels unusually hot and dry. Get into shade, cool down, drink, and get medical help if the symptoms don’t settle quickly.
Heat is a training load in itself. Pretending your 10K pace still applies is how summer training goes wrong.
Leave alone: the plan underneath
What shouldn’t change is the training plan underneath all of this — the weekly structure, the long-run progression, the easy-day mileage. Heat changes how a session is executed, not what the block is trying to build. Keep a normal-conditions baseline with the pace calculator, so that once the temperature drops you can tell the difference between “this got harder because of the heat” and “this got harder because I’m undertrained.” That comparison is the reason to keep effort-based data through a heatwave rather than writing those weeks off: it tells you, once the heat breaks, how much of the season you protected.