I have a degree in Nutrition and Food Science. It's a four-year degree, but if you already have a degree, then it will probably take 2 years to complete. If you want to be a dietitian, though, you need a year of internship to be licensed. At least, that was what they said years ago. I took the Food Science option, though, not dietetics, so I don't know much about that.
If you could get a job as an assistant to the dietitian, then you would have a ready-made internship. As far as fitness and health, I don't know. My friend went back and became a nurse. She really seems to like it.