‘Somewhat cool’ becomes ‘hot’ in a marathon
Numerous researchers have found that higher air temperatures result in slower marathon finishing times for both elite and amateur runners. The optimal air temperature lies around 8–10°C (46–50°F), with decreased performance expected to set in at around 13–15°C (55–59°F). How much slower a runner gets in warmer weather is individual and can be influenced by preparing for it.
A real-world example is the 2025 London Marathon; end of April in the UK, probably one of the first warmer days of the year. For most runners the temperatures rose from 11–20°C (52–68°F) during the marathon. Encountering such “heat” towards the end of the race when your body is not used to it saps a lot of energy out of it. Many runners were struggling.
At the Berlin Marathon at the end of September, runners faced even higher temperatures of 13-25°C. Certainly some people will have suffered from the heat. But many were more used to it from training in the warm summer months and did not suffer too much.
Thermal preparation for the marathon
Hence, 18°C (64°F) can feel hot or pleasant depending on what one is used to. You can influence this “relative heat” by doing your longer runs during warmer hours of the day (bearing in mind that you need to slow down your pace). Always doing your long run early in the morning when it’s cool is more pleasant (and usually faster), but should you encounter warmth on race day, the suffering will be greater. One study found that on average runners ran around 15% slower if the air temperature was 20°C (68°F) compared to 8°C (46°F). This results in a 3.5-hour marathoner running a 4-hour marathon.
If you live in a cool climate and race in warm conditions, deliberate heat training will prepare you for race day. By layering up and/or running on a treadmill, you can still put your body under heat stress in order to adapt. Using CORE and following a heat training protocol will help you perform better in warm conditions. You won’t slow down to as great of an extent when temperatures are above the ideal during your marathon. Furthermore, even if you get lucky to race at ideal temperatures, long-term heat training has been proven to increase hemoglobin mass, VO2max, lactate threshold, power output and blood plasma during controlled laboratory studies.
My heat training for racing in South Africa
As my first race in 2025 was Ironman South Africa at the end of March, I knew I had to prepare well for the conditions. Coming from European winter, a race in warm weather will always put extra stress on your body. From previous experience at Ironman Hawaii were I had done heat training successfully, I knew I could reduce that strain on my body. So I started the year with indoor heat training at home. I followed that up with a training camp in Spain were I trained during the warmest hours of the day and layered up when it would have been comfortable with a little less clothing. Fortunately, in March I could spend two weeks on Lanzarote were the climate was similar to what we encountered on race day.
As expected, temperatures rose to 25-26°C (78°F) during the marathon. Thanks to my preparation I did not perceive this as very hot and could run the 9th fastest marathon in a strong professional field – only 7.5% slower than my PB in a stand-alone marathon at ideal temperatures (Valencia Marathon 2023, 10-15°C).
About the author
Jamie Besse is an experienced triathlon coach based in Switzerland. During her master studies in human movement sciences and sport at ETH Zurich she founded her coaching business "Performance & Joy" together with her business partner Patrick Reindl back in 2013. In addition to coaching she races Ironman as a professional.
