I was a personal trainer for most of a decade through my mid 20's and 30's. What I learnt was my personality isn't good for mixing passion and full time work together. The secret to working with most people is to not care, when you care too much it doesn't work out well for you. Especially with anything health related, the majority of people just want to look like they're trying, so accountability is a massive no no...
All the work I do with photography is through word of mouth so I might just do one or two jobs every fortnight or what not. It's a bit of a waste having the skills but not utilising them to help others, so a bit of work makes me feel good and it also makes them feel good to get a nice product for a great price.
My normal job is mechanical engineering, I enjoy working things out and putting theories into play so it serves me well. I also still do do a bit of health and fitness work as well, mainly nutritional stuff. I have a good balance.