I started riding a year ago when I found out that I was diabetic. My doc said I needed an hour per day of vigorous aerobic exercise. My knees don't let me run, but they do let me ride. I found that riding was the about the most pleasurable thing in my life. Fresh air, exercise, no blood pressure meds, better sugar control!
Who else started because of a serious health issue? And have you become totally hooked on the experience?