to me, being a fitness professional is my full time job, like Harris.
When I look at my colleagues, though, I see that most of the group fitness instructors have fitness as a side job, and that seems to be working really well for them. Most gyms want you to teach a regular schedule, and that will enable you find a predictable balance between your work schedule and the group fitness schedule.
Theoretically, the same could apply to personal training but unless you have clients who will get out of their way to accommodate your schedule, it will be difficult for you to make the adjustments that are often necessary if clients have conflicts and need to reschedule their appointments. It could be difficult to provide continuity of training between your schedule and theirs. You also mention that you plan to go back to school to take classes. Those class times can change from semester to semester requiring readjustments.
You may be able to make something work. Frankly, I doubt whether it will rise to the level of a second ‘career’.
I wish you good luck whatever you decide, and a very happy New Year.