I'm all for walkable/bikeable cities, but that doesn't solve the intrinsic motivation problem either.
I live in an area that has a lot of walkable and bikeable things nearby. There are a lot of people who drive anyway. Some because they're older, others because they have kids, others because they have busy schedules, and some are just lazy.
So while I'm in favor of better city layout, I don't think this would be a magic solution.
> Gyms are for people who have plenty of intrinsic motivation and money and time.
There are a lot of ways to work out without a gym. You can go for a walk or run around your neighborhood or even do a lot of workout programs at home. There are many easy workout systems that don't even take a lot of time and are easy to get started if you're not in shape.
> I live in an area that has a lot of walkable and bikeable things nearby. There are a lot of people who drive anyway.
The less warm and fuzzy part of this urban-design approach is that it can't just be about making things easier to walk to, it also has to be about making them harder to drive to. For instance, by making parking limited and/or expensive. People tend not to like that idea, although I think there's a good likelihood they'd actually be happy with it if not for the meta-awareness of having "lost" parking.