In his book Heft on Wheels: A Field Guide to Doing a 180, Mike Magnuson tells about his first 38 years of eating, drinking, and smoking . He then describes his love of cycling and how he determined to change his life. At first he joined a bicycling club and began to get into shape, but since he was frequently hung-over, had smoky lungs, and weighed (in a couple of cases) double what the other members weighed, he was always far behind the pack.
As Mike changed his habits, he was able to accomplish more and more in his cycling passion. He rode several "centuries", hundred mile treks, including one called "Bridge to Bridge" which is an up-and-down mountains trail. By the end of the book, Mike had lost something over 100 lbs. and was nearly as fit as Lance Armstrong.
One major point that Mike makes is that he did not take up bike riding to get in shape. He did it because he loves to ride bicycles. In order to become good at what he loved, the other things fell by the wayside.