rather surprising suggestion 4-6 hours of mindless physical

Surprising, because the opposite of not healthy isn't bike riding all day? How about healthy died and 1/2 of good exercise? Personally I do 1.25 hrs, but that's because I read on the (stationary) bike.

As an aside, what got me into back exercise after some years was a weird medical episode where I was suspected of stroke 2 years ago. Fortunately a false alarm, but I sincerely recommend having something like that "mid-life". I'd give some of my friends fake heart attacks if I knew how.