When I moved from Philly to Boulder, it took me only 3 days to notice that everyone in CO was 'fit' as opposed to Philly, where most people are (generally) shockingly fat and a smelly city/airport. Denver can be pretty stinky, but nothing like anything south of South St. or just outside of University City. Generally, CO had more options to date people than blacking-out in some 'Irish' bar in South Jersey. That said, I liked the demeanor of people of East Pennsy and South/Central Jersey more than Boulder; they were more 'real' and didn't hide feelings or dis/likes at all. You got what you saw, while Boulder/Denver tends to be more 'shy' in demeanor and true feelings. YMMV
I can't speak for others, but I'm more fit because I live near mountains and other outdoor activities. I don't choose to live near those things because I'm fit. The causation is mostly the opposite of what you're implying.
Getting out and into nature will change your lifestyle for the better, no matter what your current abilities. And there are plenty of ways to get out there that don't require any fitness level whatsoever. Sitting in a boat and fishing, RV camping...I spent much of the past few weeks hiking in the Sierras and most of the people in our campgrounds were retirees.
;)
Philly is also a relatively short train ride away from NY, DC, Baltimore, and half a day's drive from everyplace in the Northeast.
I've been hearing a ton about this because this crew has been steadily recruiting from there to Denver.
The comments they say when they are doing this:
Philly has worse crime by far. The weather (measured in number of sunny days) is obviously worse. Outdoor activities are limited. Yeah, you can get to pretty places, but it's a few hours drive for the beach or mountains. Culturally, it's not a place that jumps out on the radar. Denver wasn't either, but the weed legalization kind of helped highlight that this place is a tolerant, open-minded location to people who automatically thought of a redneck mountain town.