Why does the body need to be trained for cardio? What does your body do when at first u can’t run 1 mile but after a while u can run 10?

Question

Why does the body need to be trained for cardio? What does your body do when at first u can’t run 1 mile but after a while u can run 10?

In:

Answer 1

None of the three answers yet actually address the question, IMHO. I’ll take a stab at it:

Your body’s constantly trying balance a bunch of finite resources. If your heart doesn’t need to pump much blood all the time, the energy to maintain the heart muscle to do that would be better spent somewhere else. Cardio training (and all other muscle training) is just telling the body that it needs to start devoting some of its resources to those muscle groups now.