Well, my #1 job is being Mom, so you could say that fitness is my secondary career. I work 14-20 hours per week in fitness.
Harris’ points are well taken. A good trainer or a good fitness instructor will make the job look easy, but it’s not easy. Obtaining and maintaining certification, keeping up with what’s happening in the industry, and preparing your classes / workouts takes time over and above the time you spent teaching in the gym.
That said, fitness can be a wonderful secondary career, particularly group exercise where people depend less on a single instructor so you can take a vacation and get subs without feeling like you’re leaving clients in a lurch. Before I left corporate America to be Mom, I worked full time and taught 2-8 classes per week.
Personal training can be done part time, but it is harder. You have to have enough availability that you can match up with your clients’ availability. You might want to set up your work schedule so you have a training niche. You could be the “early morning” trainer and as long as you can be available in the early mornings, and be prepared for your clients’ workouts, it doesn’t really matter whether you’re training during other hours. You might find that five clients, twice a week, is perfect for your case load, gives them enough attention, and you still have energy for your work and your life.