Why would you be unable to run forever without your muscles giving
out?

Well The body converts glucose to energy which produces lactic acid as a by-product. As that builds up in the muscles, it begins causing cramps and fatigue as a signal to stop. So you have to stop when you reach the lactate threshold. And Over the course of a sustained run at half-marathon pace or faster, the amount of energy (and thus oxygen) needed to maintain that pace gradually inches upward. Over the course of 10-20 minutes, your oxygen consumption can drift upward by as much as 25 per cent, making it progressively harder for you to Hold up.

I Hope this Helps! May I Have brainliest? <3