I recently crashed and burned on an attempt to PR in my past spring marathon. Racing in 75-80°F weather with high humidity is tough. Here’s a good review of running in the heat from Runner’s World.
As year-round runners facing myriad conditions, we all recognize that it’s harder to run well when it’s hot. Perceived effort is greater and race times typically suffer accordingly. Why does this happen? What happens to the body at a physiological level? And most important, how should we adjust workouts and race expectations to best weather the weather?
Source: Training in the Heat | Runner’s World