What the warm end costs
Last year's Berlin Marathon (21 September 2025) was a 09:15 start. Temperatures ranged from about 20°C/68°F at the gun to about 25°C/77°F at the finish. The course is almost free– it's flat with barely any elevation. But heat played a critical role.
For a 3-hour runner (70 kg, 178 cm) with no heat adaptation, thermal strain stacked an additional +7:17 on top of their target time.
That’s last year’s race morning. This year Berlin is a week later, 27 September. It's likely to be a little bit cooler, but even in cool conditions, thermal strain can accumulate inside one's body over the course of a marathon.
Over the last 10 years, late-September and early-October mornings around a 09:15 start are a mixed bag. Only 15% of those hours are at 10°C/50°F or below. The pile is around 15°C/59°F . The morning still warms as you run.
On a typical morning in that pile, heat costs the same 3-hour runner +2:44. Not a heatwave. But still enough to take minutes off the time your fitness earned, unless you do something about it.
A few degrees is a meaningful difference over 42.2 km/26.2 mi. Moderate heat doesn’t just add a few seconds a kilometre. It raises the chance of a big positive split, and that chance climbs fast. The heat is already eating your first-half pace and the day is still warming up. The right thing to do is back off. Holding even pace slower than your A-goal is hard for a lot of egos. Hold the original goal anyway and a small fade becomes a blow-up.
If you’re chasing a specific finish time, that difference matters. The earlier you see the heat, the more time you have to prepare.
How to avoid heat costs
You cannot control the weather. You can prepare your body for it.
Heat training, or heat acclimation, is controlled exposure over days or weeks so the body adapts. Those adaptations help you handle thermal strain and cut the heat cost on race day. Long-term heat training can even lift performance markers in cool conditions.
On that typical late-September morning, the same 3-hour runner goes from +2:44 with no adaptation to +0:43 after a standard protocol, and +0:13 after an extended one.
That’s the insurance. If race day is warm, you lose minutes instead of blowing up. And if it’s cooler, heat might not cost you anything at all.
The key is making sure you are applying enough heat stress to stimulate adaptation without compromising the rest of your marathon training. In active heat training, simply measuring ambient temperature is not enough. Thermoregulation is highly individual and the only way to ensure proper heat exposure is to monitor your internal physiology with CORE.
You can't improve what you don't measure
The CORE Thermal Sensor gives continuous, non-invasive core temperature, so you can see the thermal strain while you train.
You don’t have to go on air temperature, or on whether the session felt hot enough. CORE shows your body’s actual response.
During dedicated heat training sessions, the goal is generally to elevate and maintain your core temperature within an effective training range for a sustained period.
With CORE you can:
- Monitor your core body temperature in real time
- See how your body responds to different heat training protocols
- Avoid unnecessarily excessive heat exposure
- Track your accumulated Heat Training Load
- Measure your Passive Heat Training Load from sauna sessions to ensure proper exposure
- Follow your Heat Adaptation Score as your body adapts over time
That lets you put heat work next to the marathon plan you already have, and keep the main running sessions on track.
A sample marathon heat training approach
This is general guidance. Talk to a medical professional and a coach before you start heat training. If you get any symptoms of heat illness, stop and get help.
For a lot of athletes, the practical move is controlled heat next to the marathon plan you already have. See CORE's autumn marathon heat-training guidance to understand more.
As race day gets close, drop the extra stress and hold the adaptations you already built. CORE recommends no heat sessions in the last 5 days. A full build starts about three weeks out.
Your marathon taper should still be a taper.
The goal is to arrive better prepared, not more tired.
Don’t get caught unprepared
If you’ve spent months training, and a chunk of money on travel, entries, and fuel, heat training is insurance against a race morning that isn’t the cool one you pictured. Don’t find minutes of heat on the start line because you assumed September would be free.
Check your race in MeteoPace and see how the conditions will impact your pace. And use CORE Thermal Monitoring if you want a measured way to train some of that cost off.
