It's really hard to predict how your skin will respond to massive weight loss. While it's commonly thought that slower weight loss (with the adjustable gastric band versus gastric bypass) is easier on the skin, but the real factors are genetics and age (skin loses elasticity as we age). Exercise certainly helps tone your muscles (and burn fat), but it won't restore stretched out skin.
When I got to my goal weight, a year after my band surgery, I had some problem areas: chicken neck, a sagging belly, bat wings on my upper arms, some of sagging on my lower arms, and slightly droopy thighs. A year later, everything has firmed up a bit, as if my body had been rearranging itself. I still have the sagging belly and my neck and thighs aren't perfect but they're better. I have more muscle definition on my arms now, but I don't think that excess skin is going anywhere soon. Here's what it looks like now (20 months post-op). I'm OK with it. I'd rather have excess skin than excess weight any day.