system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually changes in your body when you build cardio endurance?

How does endurance actually work? Why do you have to actually run to gain endurance for a marathon instead of just doing strength training? What changes in your physiology when doing endurance training besides getting muscular?
gpt
Endurance comes from a process that converts glucose and oxygen into energy.

There are many steps in that conversion process, and each of them gets optimized in a certain way.

A top endurance athlete will have more red blood cells to carry oxygen, a bigger heart muscle to pump the blood faster, and more mitochondria (the little furnaces that combine glucose and oxygen) inside each cell.

There are other adjustments as well, but those three are the most important. Note that none of these adjustments involve any physical change in the size of the muscles, other than the heart.

As for strength training, that primarily involves an energy pathway that does NOT use oxygen. Bigger muscles will not help much with endurance, as long as the athlete has enough basic strength to keep the body stable. Otherwise, big muscles just end up as added weight and bulk that can slow down an endurance athlete.