Be sure to investigate the recumbents; they are the most comfortable of bikes. Like you, I have a very bad back. I'm 53, 5'11", 213 lbs. I did not think my back would let me seriously ride a bike at all until a recumbent stationary bike convinced me I might be able to ride a real recumbent bicycle. Now I ride 80 to 100 miles a week, really pushing myself one or two of those rides each week, and it doesn't hurt my back at all. Come to think of it, I don't hurt anywhere from riding my bent.