>>12854581O2 feed rates and acceleration windows.
You body often uses O2 to help power your motor functions, like running.
In a sprint you body quickly uses all the O2 faster than your blood can resupply, thus you body switches to mostly Anaerobic. Running like this causes acid to break down your muscles, causing the new muscles to grow back bigger and stronger to help deal with the repeated abuse. In high level sprint the muscle don't have time for the heart to supply more O2 so they do other things to force themselves to keep going beyond normal limits. The body is built to handle this to some level, so some sprinting is good for you. Also the more muscles allows for faster acceleration which is key to winning sprints, so muscle building is part of training as the added weight of the muscles has less impact over such short time windows. Sprint is muscle focused.
In a marathon it is completely different. Because the time window is so long the methods for sprinting are counter productive. Running for to long without O2 will force the muscles to shutdown, and going fast then stopping is noway to win a marathon. The weight of the extra muscle just makes more work and will cost more the longer the run. So marathon instead focus on better O2 management so the runner can maintain elevated output for longer times, thus a Marathon is cardio focused. The human body is one of the best marathon platforms, it can out range other land animals as we can keep a light run going for hour and hours. Long after others have been passed by with our speed or faster animals have collapsed from exhaustion. Marathon running have much better long term benefits as it seems humans were built for marathons
I hope you find this scientific rambling helpful.
