The key is to vary the type of exercises you do, so that you do not get bored. Try and gradually increase the volume of exercises you do. You need to get your heart rate up.
WATER JOGGING :
Running in the water is an excellent way of improving your conditioning. Do it in shallow water in the swimming pool. Keeping the back straight. Begin gently by just walking fast in the water.
Move on to jogging in the water. If you have a buoyancy vest you can do it in 8 feet of water and run on the spot. You could also join a water aerobics class.
Gradually improve your speed and duration of jogging.
You need to be a good swimmer to swim without straining your back. Avoid the breaststroke, because it arches your back. The crawl or the backstroke is quite good. Again, begin slowly and increase the duration you swim.
An exercise bike is an excellent all-weather way of keeping aerobically fit. The semi-reclining bike supports your back, which is better for your back.
While walking is good for you, you do need to walk fast to get a sweat up and get aerobically fit.
Use a short rope and do a 2-step jump. Count the number of jumps. Progress to an alternate step jump and eventually to a shorter rope. Begin with 15-second intervals. Gradually increase the number of jumps. This may not suit all patients.
With all the above activities keep a chart and try and improve your fitness on a weekly basis. Begin gently and slowly. Keep within your pain tolerance. Increase the volume of exercises gradually.