Need Help

For the last couple of years I was extremely active. I was jogging about 10miles a week, lifing 4 times a week, and well as playing soccer every Sunday. Last month I had some lower back pain and visited the doctor. I recieved horrible news that I can no longer jog, play soccer, or lift weights because of a herniated disk as well as some other things. She told me that if I want to maintain my fitness I should swim. I feel great now and the doctor said that if I went back to my normal routine I may be ok for a while, but the problems would re-occur. I am not interested in ruining myself. But want to keep the fitness level I was at or increase it if possible. I am not a swimmer like most of you are, but I have enjoyed recreational swimming all my life, I am 27 years old 5'7", 170LBS, and 12%BF. What kind of fitness workout would you guys recommend. The strokes I am capable of are *** & freestyle. I feel very awkward doing the others. What kind of things would you recommend for me. I can swin 6 days per week for 50 minutes each time. I really want to burn fat and maintain (or improve) my body.