It's possible that I'm in your grandparent's generation, or maybe your parents. I'm 65. I don't want to derail the conversation, but I do want to correct the bit about "exercise as a hobby was viewed as extremely weird, runners used to be arrested!". In the '70s at least, running was extremely normal. Endurance bicycling was normal. For instance, starting in 1979. I bicycled all over the South: FL GA NC, at least. These were then as they are now quite provincial from the POV of alternate ideas. 10K runs were a thing, with thousands of participants. As were Marathons. Cross country running was a popular high school sport.

If you want to talk about work, well I'm adjacent to 5th generation farmers. Self driving climate controlled machinery is a thing now, but! hard to imagine I know, people had tractors back then too.

So, plausible as it might sound to ascribe some of the causality to flow from "the physical activity transition", I don't think it holds up.