My first rule is ALWAYS hold your GI's opinions/advice over a primary physician's. They have much more experience and training with this disease.
The general consensus is that your risk of colon cancer increases once you've had UC for 10 years. It also increases if you've had the disease since childhood. However, the overall percentage is still pretty low.
As stated before, after 10 years, you should begin to get frequent colonoscopies to monitor the health of your colon. Earlier detection of colon cancer is important.
Surgery sometimes involves removing the colon. Some people opt for a j-pouch, which is a pouch that's surgically connected to your digestive system and housed inside your body. You empty via normal bowel movement method.
Don't get too far ahead of yourself. I have very mild colitis and my doc has mentioned the surgery and cancer as well. It's definitely worthy of your awareness, but nothing to freak out about.