I'm 53; degenerative discs have been my biggest health issue for the last 8 years. I'll second the recumbent bike suggestion. I cannot find a sitting up position that will work for me for any length of time, even on a bar stool, but 80 to 90 miles a week on my recumbent bike doesn't hurt my back at all.

One thing I have learned to guard against while riding: when you are really pushing it on the bike, there is a tendency to tense up all over, even the muscles you aren't using, including back muscles. If I let myself do that for any length of time, my back will hurt. These days, whenever I'm riding hard, I make sure to do all the work with my legs, and keep my back relaxed.

While the recumbent bike doesn't seem to hurt my back, it doesn't help it, either. One thing I have found that does seem to help is exercising and stretching with a video from the University of Michigan Health System called Health Abs and Back. You can get it online for just $11, including shipping, at: http://www.med.umich.edu/pmr/spine/video.htm. I do the exercises after every ride (four days a week).