As in any sport, too much too soon is a setup for a certain set of injuries. For cycling, saddle sores, IT band syndrome, knee injuries from ill-fitting pedals, low back pain, and flexibility issues come to mind.

It's good that you're already active, but once you change the motions/muscle groups involved, there's a risk of injury if you don't get your body accustomed to it. When I added running into my training, for instance, I added too much speed/hill work too soon and got achilles teninitis--minor, but miserable. I'd never had any achilles issues in 10 years of road cycling.