Please can someone tell me the best way for me to get fit. I used to be pretty good, I'd run 10k 3x a week, I did a lot of swimming, some weeks I'd go 6 days a week for an hour each time - proper head in the water, front crawl, out of breath swimming.
After years of silly injuries I was diagnosed as having hypermobility disorder, so I'm guessing high impact exercise is out. I've torn my cruciate ligament in my knees a total of 4 times, I've got almost no cartilage left in them. My back is shot, with a damaged vertebrae. I've dislocated my shoulder a couple of times too 
I'm currently recovering from an operation to repair yet more stupid injuries, 2 ruptured tendons and a broken bone in my foot, but as soon as I'm able to walk properly I really want to get serious about getting fit.
I'm 44 next month and I don't want to end up like my grandmother, never doing a days exercise in her life and then crippled by stupid falls because she had no core strength by the time she got to her 60s.
I'm assuming swimming is my best bet, but I'm open to any ideas. Is there an equivalent to couch to 5k for swimming? I think if I try swimming I need a programme to follow.
Please help me so I don't become a stooped little old lady who breaks her hips just walking up the stairs! 