[Rizz]

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.