Recovery Time
In high school, I would swim 11 times a week, two hours a workout. Now I swim 3 times a week for a little over an hour and lift 3 times a week.
During high school, I remember being tired, but I was working out over 40 hours a week. Now, I work out at most six hours a week, and I am regularly physically exhausted.
I just wanted to point out how I realized I was getting old.