Several years ago I injured my lumbar spine at the gym. As a result of that injury I have been unable to do, well, anything for about 3.5 years now. Needless to say, any sort of physical fitness I had at the time, which was fairly limited to begin with, has long since gone out the window.
I recently underwent a procedure that actually seems to be working. I am in less pain and I am more physically able now than I have been at any point since the initial injury. On my doctor's advice, I am slowly getting back into fitness training to try to recover some of what I lost. In particular, I've started swimming regularly. However, even after several months of doing it consistently, my endurance does not seem to be increasing even slightly. I am wondering what I can do to maximize the impact of my "workouts" given that my goal in the short term is to increase my general stamina.
A few things to keep in mind:
I cannot lift anything heavier than ~8 lbs without causing further damage to my spine. This number is steadily improving (up from about 4lbs!), but weight training is simply not an option yet.
I can swim. I can do about 250 meters at this point before my lungs give out, and I'd like to improve on that. Swimming does my back no harm at all, so the limiting factor there is my general stamina, not my injury.
I can do a limited set of core body-weight exercises. These are going reasonably well and I am noticing some improvement there in terms of what I can do. I also have a very good sense of what my injury can and cannot tolerate in this area, so I am generally able to know ahead of time what is and is not safe.
I can use an exercise bike, and I am starting (slowly) to experiment with light jogging. No more than a minute at a time right now, but so far it has done no harm so I can probably start to increase that. If things continue to improve I am considering purchasing a regular bike this summer.
What can I do (workout details, diet, lifestyle, etc) to maximize the impact of the exercises I can do, given that my immediate goal is to improve my endurance in the short term, and to lose a small amount of weight in the medium term? You can rest assured when giving advice that I am well aware of the possibility of further injury and I will be very careful when implementing any suggestions.