system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When muscular endurance is increased, if muscles are not getting larger, in what way are they becoming stronger?
gpt
They are getting more blood flow and more mitochondria.

This allows for even more aerobic respiration (burning oxygen) which is sustainable long term unlike anaerobic (making lactic acid) you use when weightlifting. Mitochondria are also where fat is metabolized and burned so they will be able to tap into their fat reserves to a greater extent than a non-endurance athlete would. For blood flow, the muscles will get more vessels supplying them while at the same time the amount of blood volume will increase and there will be more total red blood cells/hemoglobin to deliver the oxygen. Waste products can more rapidly be removed and metabolized.

It’s not about strength, it’s about being able to function for hours without rest.