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Why Your Heart Rate Climbs Late in a Half Marathon (And What to Do About It)

"I'm a fit 25-year-old with a body mass index (BMI) of 22, and I've been running for two years. I've estimated my maximum heart rate at 205 beats per minute (bpm). When I run at around 6 minutes per kilometre — talking pace for me — my heart rate sits between 175 and 185. My training partners are similar in size and fitness, but their heart rates sit between 140 and 150 at the same pace. In my past half marathons my heart rate has stayed over 180 for the entire race, and I generally finish the last 3 kilometres over 200 bpm, even though my pace slows. I hit the wall at around the 18km mark, and I want to work up to a full marathon — but with this happening, it feels impossible. Is my heart rate the problem, and if so, what can I do about it? I don't feel like I'm running fast, so I don't want to drop my pace. Thoughts would be appreciated.

– Evie, Melbourne

Answer by Dominique: Hi Evie,
Thanks for your question about your heart rate climbing so high on long, steady runs.

If your heart rate climbs steadily through a long run or race even though your pace hasn't changed, that's cardiac drift — a normal response to rising core temperature, fluid loss, and glycogen depletion. It isn't a sign something's wrong. For most runners it's manageable with pacing, hydration, and fuelling, and it's genuinely rare for it to signal a medical problem.

I'll break my response down as follows:

1. What's Actually Happening: Cardiac Drift Explained
2. Is Your Heart Rate Actually a Problem?
3. The Real Levers: Hydration, Fuelling, and Pacing
4. Building Endurance to Push Past the Wall
5. When a High Heart Rate Is Actually Worth Checking Out
6. Bottom Line for Your Training

1. What's Actually Happening: Cardiac Drift Explained

Let's name what's actually going on, Evie. What you're describing is called cardiac drift — heart rate rising over time even though your pace and effort haven't changed. If you want the fuller picture beyond this one page, I've written a full guide to heart rate training that covers the basics.

It happens because your core temperature climbs the longer you run, and sweat loss drops your plasma volume — the fluid portion of your blood. Your heart then has to work harder to push the same amount of blood around your body, because some of it is being diverted to your skin to help you cool down. Same output, more effort — that's cardiac drift in a sentence. Drift is worse the longer the effort goes on, and worse again in heat and humidity, or when you're under-fuelled or under-hydrated. That maps almost exactly onto your pattern: your heart rate climbs specifically in the final 3 kilometres of a half marathon, right when heat build-up and glycogen depletion are both compounding at once. That's not a random spike — it's the mechanism doing exactly what it does, later in the race, when the conditions for it are most stacked against you.

2. Is Your Heart Rate Actually a Problem?

Let's do the maths on your actual numbers, Evie. At 175–185 bpm against an estimated max of 205, you're training at roughly 85–90% of max heart rate — on runs you're describing as talking pace. That's worth a caveat: a self-estimated max heart rate, using something like the 220-minus-age formula, is frequently wrong by 10 to 15 bpm or more, in either direction. If you want a more reliable number to work from, it's worth trying to get a more accurate max heart rate estimate rather than relying on a formula alone — your real percentage could be meaningfully different to the one you've calculated. Finishing a half marathon over 200 bpm isn't, on its own, a red flag. Provided that's close to your true maximum and isn't accompanied by chest pain, dizziness, or unusual breathlessness, it's consistent with an all-out effort — not a warning sign. And here's the thing about comparing yourself to your training partners running at 140–150 at the same pace: it tells you almost nothing. Max heart rate and aerobic fitness both vary substantially between individuals. The comparison itself is the flawed instinct here, Evie — not your numbers.

3. The Real Levers: Hydration, Fuelling, and Pacing

You've told me you don't want to drop your pace — so let's talk about the levers that reduce drift without necessarily slowing you down.

Even, disciplined pacing from the start matters more than you'd think. Going out too fast burns through glycogen and raises your core temperature earlier in the race, which accelerates drift later on. Proactive hydration and electrolytes — not just drinking when you're thirsty — matter too, because fluid loss is a direct driver of drift. Thirst lags behind your actual fluid deficit, so by the time you feel it, you're already behind.

Over efforts longer than about 75 to 90 minutes, carbohydrate intake during the race helps delay glycogen depletion — which is exactly the kind of compounding effect showing up in your final 3km. And in hot or humid conditions, drift is more pronounced for the same effort, so pace a little more conservatively and hydrate more aggressively than you would on a cool day.

4. Building Endurance to Push Past the Wall

Now let's deal with that wall at 18km. Push your long run up toward something in the region of half-marathon distance — or a bit beyond — before you race one, so 18km stops being unfamiliar territory on race day. Your body handles distance it has already covered far better than distance it's never seen. Practise your race-day fuelling and hydration on those long runs too, not just on race day itself. That way your body has already adapted to taking on fluid and carbohydrate mid-run, rather than trying it for the first time under race pressure.

For now, get through one or two half marathons comfortably at your current volume before you layer in marathon training. Marathon training will ask for even longer long runs again, so half-marathon comfort first is the sensible build-up — not a delay.

5. When a High Heart Rate Is Actually Worth Checking Out

Let's be clear about what's normal and what isn't, because this matters more than any pace advice. Normal looks like this: your heart rate rises with effort and comes back down within a few minutes of easing off, you have no symptoms during easy, conversational-pace running, and you feel recovered within an hour or so of finishing.

See a doctor promptly if you notice chest pain or pressure, dizziness or fainting, palpitations or a skipped-beat sensation, or breathlessness at an effort that's normally easy for you — especially if there's a family history of heart disease.

I want to be honest about the limits of this, Evie: this is triage, not diagnosis. I'm not in a position to rule anything out from a forum post. If a persistently high number during easy runs is what's actually nagging at you, separate from race-day drift, it may be worth asking could this be overtraining instead? And if it's a more general worry about long-term heart health rather than this specific pattern, I've covered that separately here: worried a high heart rate is bad for your heart?

6. Bottom Line for Your Training

So, Evie — here's the bottom line. Your heart rate pattern looks like normal cardiac drift, compounded by race-day intensity. It isn't a warning sign. Keep your current pace, tighten up your pacing discipline and your hydration and fuelling, and extend your long run toward half-marathon distance to push that 18km wall further out.

Get comfortable finishing half marathons at this volume first. The marathon will ask more of exactly the same systems — heart rate, hydration, fuelling — so get good at managing them over 21km before you ask them to do it over 42km.

Kind regards,
Dominique

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