If you are entrepreneurial, you can actually do quite well as a personal trainer. I am not in that industry (frankly, I think I would go crazy with boredom), but I am an avid powerlifter and know quite a few people in it, including the owner of the gym I frequent. I know for a fact that with only 50 members and a handful of regular personal training clients, he is doing very well for himself and gives exactly zero fucks about anything, conducting every aspect of his business exactly how he wants to. I've personally seen him fire two clients (loudly and publicly) because they were being obnoxious (overly questioning him and/or being dicks to other members), and I know they are not the only ones.
I've also heard of in-home trainers (that will visit you at your house) who have done very well. I'm sure others here have more experience than I, but my impression is that if you can thoroughly learn the boring technical aspects of training (body mechanics, dealing with injuries, etc.), have a great attitude towards customer service, and don't mind the repetitive monotony of coaching newbies, personal training can be quite rewarding and relatively lucrative.