I'm nowhere near the level where these people compete, and my thing is road biking, not running, but I did notice the best day for a ride up and down the hills is: low temperature, dense clouds, maybe even light intermittent rain (barring any traction issues in corners).

BTW, strength-based sports also take a toll on the body. Maybe even more than long distance running. You can keep doing long races well into advanced age, whereas weight lifting after 40 is a bit sketchy.