Mr Silver - (Sorry if this has been posted elsewhere before but...) If you cannot run, absolutely find some sort of weight training plan to supplement your cardio.

There has been many studies recently about the prevalence of osteo degeneration among cyclists... turns out calcium is lost in sweat as well as sodium. Just google "osteoporosis" and "cycling" and you'll get a ton of hits.

From what I've heard, it's best to supplement your cardio with some kind of impact exercise (weights, etc), and add 200mg of calcium for every hour of cardio exercise.