Above ~18 °C, marathon performance starts dropping. Acclimatisation, pace adjustment and hydration strategy can win back minutes.
The pace tax
Rule of thumb: add 2–3 seconds per kilometre for every 5 °C above 12 °C. A 22 °C marathon costs roughly 6–9 seconds/km versus ideal.
Race effort, not pace. Heart rate is more honest than your watch on hot days.
Two-week acclimatisation
If your race is in the heat, do 10–14 days of heat exposure beforehand. Sauna sessions of 25–30 minutes post-easy-run are highly effective if outdoor heat isn't available.