system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the body get ‘used to’ running long distances?
gpt
Your body gets stronger and through that it also gets more efficient at using the oxygen you inhale.  As a result your heart doesn’t need to pump as much blood throughout your body and you don’t need to breath as hard/heavy.

Basically you aren’t as strained physically due to lack of oxygen, because your body needs less oxygen to do what it needs to.