I am of the opinion that gyms can be a good compliment to a plan of getting into hiking shape, but it isn't wise to rely totally on them. I personally can't stand going to them other than to check out the hotties. The cookie cutter exercises there and normal hikes down trails just don't work muscles at all angles IMO.
Personally, I trap and bird hunt all winter long on foot then bear hunt and fish in hard to reach areas all spring/summer. Those in the deep south may be able to find some swampy mess to get through and fish which I suspect would be as taxing on your legs, hips, back and core as anything else.
