i quit smoking 2 and a half years ago, and i gained weight because i was pregnant, and breastfeeding but i suspect its also the fact that i no longer smoke that increases my weight as well.
Apparantly it decreases the metabolism by 15% on average.
I take comfort in the fact a little extra weight is a lot more attractive than wrinkles around the mouth, yellow/brown teeth, hacking, that ashtray smell, possible cancer, chemo, hole in my trachea....etc....
eventually your lung capacity will be able to keep up with your new exercise requirements, i almost cant tell i ever smoked, i still cant run, but i work out everyday without gasping for air.
hang in there!....and check out about.com,,, the smoking cessation forum is fantastic!