Strava Reports 50% Bonk Rate at 2025 London Marathon Amid Unseasonable Heat

The 2025 London Marathon, held on April 27, witnessed a significant slowdown among runners, with over half experiencing a “bonk“—a term used when athletes slow down by 10% or more in the second half (Claim: Strava). This data, provided by Strava, the global fitness tracking platform with over 150 million users, highlights the challenges participants face this year.​

Heatwave Hits the Marathon

A primary factor contributing to the increased bonk rate was the unseasonably warm weather and likely poor thermoregulation by many inexperienced athletes. Race day temperatures peaked at approximately 22°C to 23°C (71°F to 73°F), a notable rise from the previous year’s cooler conditions, which ranged between 7.3°C and 11.1°C (45°F to 52°F). The Met Office had forecast these warmer conditions, advising runners to prepare accordingly.​

As you can see from the table above, in these conditions, runners of the 3-hour standard would need to adjust their target pace by 6 seconds/km.

Sensible tech-lovers of this site might also have used a Core 2 Body temperature sensor and kept their efforts in Zone 2, like Kristian Blummenfelt recently won in Texas.

https://www.instagram.com/p/DI_GMeCIQJW/

Performance Metrics Decline

Strava’s analysis revealed that average finishing times in 2025 were slower across all demographics compared to 2024. Specifically:​

• Women: On average, female runners completed the marathon over ten minutes slower than the previous year.​
• Men: Male participants averaged finish times exceeding 4 hours, compared to 3 and 53 minutes in 2024. (at least 6 minutes slower)
• Generational Impact: Millennials and Generation X runners experienced the most significant decline, averaging 12 minutes slower, while Generation Z and Baby Boomers were approximately 10 minutes slower.​

The 5th kilometre emerged as the fastest race segment, with an average pace of 5:20 minutes per kilometre. This indicates that many runners started strong before succumbing to the heat in later stages.​

Compare those declines in performance to the declines indicated above due to temperature. I contend that better temperature-adjusted pacing would have meant some athletes were only 3 minutes slower than hoped, rather than the 6 minutes we saw.

Elite British Performances

Despite the challenging conditions, several British athletes delivered noteworthy performances:​

• Mahamed Mahamed: The top British finisher in the men’s elite race, Mahamed secured 9th place with a time of 2:08:52.
• Alex Yee: Making his marathon debut, Olympic triathlon champion Alex Yee finished 14th overall with a time of 2:11:08. Yee started with a strong first half, clocking 1:04:19, and was paced through the initial 30km by fellow British triathlete Hugo Milner. Despite facing physical challenges in the latter stages, Yee was pleased with his performance.
• Eilish McColgan: In the women’s elite race, McColgan finished 8th, setting a new Scottish women’s marathon record with a time of 2:24:25. This performance surpassed the previous national record held by Stephanie Twell and bettered the personal best of her mother, Liz McColgan, who won the London Marathon in 1996.