Via The Art of Manliness I found this great graphic of a nature pyramid (à la the food pyramid) at the Nature Kids Institute. It’s aimed at kids, but really the prescription for adults should be exactly the same.
It’s tougher in the winter, of course—when the grass is snow-covered and the paths are icy, they simply aren’t so runable or walkable. But that’s okay. It’s still worth getting outside.
It’s also tougher in Illinois to find wilderness than in most other places I’ve lived. There was real wilderness in Michigan and Indiana and Florida (although when I lived in Florida I did a pretty poor job of spending time in it), and of course vast amounts of real wilderness in Utah and California. Still, there are places in Champaign county where I can at least get out of sight and out of earshot of roads, even if it isn’t really free of human influence.
Anyway, these quantities seem like good initial target minimums for time in nature. Maybe not optimal, but adequate, and probably a lot better than most people manage.