My first PG I weighed 292 when I got preggers and only gained about 15 pounds. My 2nd PG, I was 317 when I got pg and delivered at 304. This one I was 285 (naked at home) and am up to 296 (fully clothed with shoes on--I think they have me starting out at 290 at the doc's office though). It's possible for you to not gain a whole lot of weight this time and possibly even lose a few pounds.
I do have the gest. diabetes and give myself 4 shots a day. I had it last PG too and neither time has it been too bad. I have a great doc and a superb diabetic consultant and I think that makes a big difference. Yes I hate the monthly pee in a jug to check my creatinine levels, the 4x a day shots, the numerous sticks to my fingers daily, but I do get extra u/s scans and it seems like more care than your regular PG.
As for the exercise, just try and do what you can and don't get too discouraged if you are just too tired. Aphil over at the PG/nursing forum is still belly dancing I believe and is 30 weeks.