I'm so glad you've gotten so many responses! I know I'm a little late to the game, but I'll offer my two cents, anyway. I had my surgery done when I was 21 (I'm 22 now). I did it during winter break and missed the first week of class because walking tired me out too much. Plus, my bladder was minorly punctured during surgery, so I had a catheter and I my JP in after being released from the hospital. I didn't want to go to class like that, on top of having to be wheeled around by my mother.
I went from 115 to 97 in the hospital, but I gained it back with diet and exercise. My weight has been stable and I haven't been going to the gym, no issues losing or gaining.
It took me about two months before dancing the Cupid Shuffle didn't completely wind me, if that's good for comparison. Regular exercise probably didn't resume for four or five months, and even then it was very, very light exercise. Walking laps, mostly. I waited for my scars to get closer to my natural skin tone, so I knew everything under the surface was more or less fully healed.
Stomach muscles: It hasn't effected mine, really. I stretch regularly to make sure the scar tissue isn't too tight. Around the six month mark, if I went a week without stretching out a little bit, my stomach muscles would feel very tight, but I haven't noticed any long-term effects. It's going to take a long time to get your stomach muscles back to where they were, though. They got sliced all the way through. Don't rush the process. I think it was about a year for me before I could do 20 sit-ups in a row (but then, I also wasn't too fit before I went in for surgery).
As for demoed tumors, every body is different, both with possible future cancers and how the body responds to certain foods. My mom has issues with mushrooms; I have issues with corn. She has issues with dairy, too, and they haven't found polyps in any of her colonoscopies for the last twenty years, but they found two in my second colonoscopy. I'm not concerned about it. The doctor will tell me if I need to worry. It sounds like your doctor just wants you to be aware that removing the colon might not solve everything. It solves the most important thing, the most pressing thing, and that's the thousands of polyps that will eventually turn cancerous, all lining your colon. Other than that, it's up in the air what happens next. Just pay attention to how your new digestive tract interacts with your body. If you're good about your yearly colonoscopies, you shouldn't have anything to worry about. They're good at catching these things. Make sure, though, that they're checking out as much of your digestive tract as possible; FAP affects the thyroid, too, and the esophagus and--I mean, literally the entire digestive tract. If they're only giving you yearly colonoscopies, they might not understand enough about FAP, so you might want to consider seeing someone with more genetic colorectal cancer experience.
Lastly, you'll be good by August! 
I was in class about four weeks after my surgery. It wasn't the most comfortable experience of my life, and I was walking veeeery slowly, but it wasn't so bad.