My friend is a PT, he charges $100 per session, dude has weird hours but he's his own boss. He started at NYSC then got his client base up and went from there, does house calls, in gym stuff and works out in central park with people.
Last I asked him he was making close to $100k, but that is in NYC. And, just like you he played football in college, seems like the most logical path after the college career is done.