To be physically fit is just a small aspect. You can be a beautiful physical specimen, but you're empty as far as what it takes to be a person, and that shows up real fast.
I had two family members involved in World War I: two great-uncles. One of them is on a memorial in France. And the other was a trench runner who survived the war. The average life span of a trench runner was 36 hours, but he survived the whole war.