Set up a training schedule. Half the days you ride, do stuff that sucks. I don't need to tell you what- you know... long, steep climbs that have you straining out of the saddle at the end doing like 2 miles an hour, extended sprints on cobbles, etc. The other days, go for long fun rides. The first few weeks your "free" days will be slow and easy, like recovery rides... but as you get into better and better shape, your free days will become more and more challenging... soon you'll be killing hills for fun. Then you can start eating people. Mmmm... people.