Correlation does not imply causation
Actually, there is causation.
City dwellers walk more than suburb dwellers, and are thus in better condition. Of course, once upon a time, the opposite was the case. Studies done by the army in WWI showed that country dwellers were in much better shape physically than city dwellers (farm work builds muscle!).
So if you REALLY want to get in shape, live in the country, and adopt the lifestyle of a farmer from 1900.