I started my weight loss challenge weighing 333.6lb. I decided to buy a recumbent exercise bike to help me lose weight. At first I could barely complete 10 mins, but in just 2 weeks I can now ride 11 miles in 30 minutes.
Start slow and do what you can. You will be amazed at how quickly can quickly you can build endurance and lose weight by using an exercise bike!I have lost 30lbs now in 6 weeks
Try a Recumbent if you are worried about your lower back, the support they provide is wonderful.