personal training is good when your young and full of energy. but when you get older and life gets serious constantly hustling for more clients gets old. Its not a long term stable career. also when you get old, who would want to hire you? you can only rely on your looks and body for so long. think about it that way.
That all holds true if you are the stereotypical trainer, which most are, sadly. I've made a career out of it for 21 years as well as owning a gym for the past 9. It's knowledge and the implementation, and application of that knowledge to the particular clients which is what will set you apart. Anyone can count 3 sets of 10 reps, discuss their personal problems, ask for money, and model their physiques for "clients", but that's not what the job is when you do it on any kind of advanced level, which is the only way to have a sustainable, as well as interesting career.
I train several doctors, a host of athletes, pros and hopefuls, a lot of phys therapy clients, as well as some decently known entertainment industry people. I don't say this in an effort to impress anyone whatsoever, for even a minute, but simply to point out, that if you apply yourself as a student of the craft, learn anatomy, physiology, kinisiology, movement, nutrition, etc,, etc, and are able to apply all of it to every INDIVIDUAL client, you'll be on to something.