Great comments from coach — I’d also note that it’s not JUST heat that matters, it’s humidity/dew point. Several mornings last week had relatively bearable temps (75-78 degrees), but added dew points above 70 (note that dew point is different from humidity measurements and is a better tool for adjusting exercise; both are available on many weather websites). That sort of a combination means that your body will generate heat, but your sweat won’t evaporate well enough to cool you. This gets really dangerous during a run as your internal body temps will rise throughout, and it becomes a real challenge to stay hydrated and not overheated. So, as coach notes, stay hydrated, ensure you’re consuming electrolytes (and not just sugar), and SLOW down.
Below’s a link to a table I like to refer to to adjust my run paces in high temp + dew point conditions. In a day like I described a above (78 degrees + 72 dew point), we should be dropping planned paces by about 5 percent — for a 10 min/mile plan, that means you’re looking to run 10:30/mile or even slower as you go longer… For type A, wanna-go-faster-every-workout folks like me, it’s really hard to get this through my thick skull!