Marathon and other endurance sports are interesting.
It takes a toll on the body. It catabolizes it. Compare a marathon runner with a 100m runner.
Also, as expected, temperature plays a big factor in it, which makes me wonder why so many people "love when it's a nice day" (read: sunny and hot) to exercise.
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.
Always found that strange as well. I've always loved being a runner in Seattle because its generally cool, cloudy, and often light rain. To me that's the perfect weather for a run since it prevents you from overheating or dehydrating as quickly, and, in my experience, the sun tires you out much quicker. I've always been confused about the fact that you see by far the most people out running on a hot, sunny day. Every one of them seems to be sweating bullets.
A good rule of thumb is to assume that the temperature is going to feel 10 degrees (F) warmer when you're running or doing extended aerobic exercises outside.
Because sunny weather improves peoples mood, just look around you when it's raining and cold, people are glum and looking down a lot, while on a sunny day you see plenty of people smiling. Good mood means you feel better and perform better.
Second, your muscles are already somewhat warm, so you feel powerful right from the start instead of having to warm up for ages before you're ready for top performance.
Also, your training or just going for a run is not at the same intensity as running a marathon as a race. You run shorter distances or lower intensity, meaning the stable body temperature at marathon pace and distance is a cooling body at training pace at the same outside temperature.
People like to sweat and believe sweating is the key to a good workout. When it is hot, you sweat easily. Most people who love to exercise when it is sunny and hot aren't going out for 20 mile runs.
Anyone into running will tell you their favorite days are overcast, cool, and even a little drizzle doesn't hurt. Hell, my favorite time to run is the winter.
It takes a toll on the body. It catabolizes it. Compare a marathon runner with a 100m runner.
Also, as expected, temperature plays a big factor in it, which makes me wonder why so many people "love when it's a nice day" (read: sunny and hot) to exercise.