Limit zoning restrictions that currently make driving everywhere the most convenient choice, stop all freeway expansion projects, and make as many walkable, bikeable, friendly neighborhoods as possible.
If people want/need to go outside and walk or bike to accomplish their daily chores, they won’t need to think about exercise as much because it’ll just be an automatic part of their routine. The best kind of health outcomes are based on societal changes, not personal responsibility (“you need to go to the gym and punish yourself” has been failing us). And people will be happier too, if they are more connected to their community (after the initial grumbling about how “hard” it is). It’s so hard to walk/bike around in the US that I don’t blame people for not doing it more (as an avid walk/run/biker myself).
That doesn’t mean we get rid of all current development and suburbs, but just eliminate the current restrictions which enforce that lifestyle, and let people’s communities start transforming slowly into healthier ones.