A quick scan of periodical shelves seems to indicate every other mag is devoted to either diet or fitness or both. I certainly think many of the fitness and/or outdoor-related could be receptive to the proper pitch. A how-to-get-started article could get your toe in the door but you'd have to search back issues to ensure it hasn't been done by that particular periodical recently.
As to specific markets, you might start check out
Outside magazine for starters. I'm not familiar enough with the fitness markets to suggest others but a visit to your library or large mag retailer should provide you with some leads.
Good luck.