Research what kinds of certifications are available to you, these are professional credentials you can get relatively cheap compared to a full degree. There are numerous available, some that deal with anatomy or physiology, nutrition etc.
Pick up some certs, and start a fitness troupe. Recruit people from the gym or the super market, put ads online. I see people all over the city throw a logo on their car and drive around to meet people or groups and provide personal training. This could be you.