This is a great question Vince, and a very clear point. As I’ve heard people say before, it has the virtue of being true.
In my experience, extreme physical activity is counterintuitively restful for the mind. Our conscious mind is like Chester in this clip:
We are always interrupting the important work our subconscious is doing keeping us sane and healthy.
When we do something that taxes us physically, even for a half hour, we can’t help but focus all of our attention on it. Doing that gives our subconscious a much needed break to continue working out the important stuff.