It's the "forcing yourself to do something unpleasant" part which improves mental state, not the fitness itself. I suspect that people who find exercise enjoyable, or are forced to do it, don't get the same benefits.

This article is basically just the marshmallow test: https://en.wikipedia.org/wiki/Stanford_marshmallow_experimen...

And 4-6 hours a week would be a low week for me =)