Surely the exercise that comes with using public transport outweighs that. I feel like being able to zone out and not having to focus on driving, is more relaxing to me.

Yup, public transit over driving for me any day. I grew up in NYC which has a relatively nasty public transit system too.

If we're talking about the extremely elderly, it seems better for them to be taking public transport than driving.

The extremely elderly are staying at home or a nursing home, and occasionally and delicately transported by car.

But lots of walking in their less elderly days may partially explain their longevity.

Which doesn't track. Public transport in Japan is peaceful. I feel more relaxed on a Train in Japan than a car or Train in Canada.