Sort of nitpicky but obesity does affect you in aggregate because of increased burden on the healthcare system that is more or less subsidized by healthier folks. It’s never really better to have more people subjecting themselves to avoidable health problems than otherwise.

Doesn’t seem to be strictly true, but I do think it’s complicated. Running is awful for your knees if they’re injured or you have some sort of congenital skeletal issue. If you’re just sedentary, it’s awful for your knees if you do too much too soon.

Otherwise it seems to strengthen them. I will grant that it’s pretty hard to truly run easy enough and build up slowly enough. Takes patience and some trial and error in my experience. I skateboarded through my teens and 20s and had all sorts of nagging pains that have gone away completely after a couple years of consistent running, as an anecdote.

https://longevity.stanford.edu/lifestyle/2023/08/29/is-running-bad-for-your-knees-research-says-no/