i am in EXACTLY the same boat... well, ok, except for the being thin part. i had twins 5 years ago, and yes, the skin on my tummy is destroyed... there's no other way to describe it. as i lose weight, i'm noticing the demarkation more and more between just the fat parts of my torso and the parts that were pregnancy-related. i'll eventually have to have a tummy tuck to get rid of it... i came to that conclusion long ago.

one thing you didn't mention... how old is your baby? if we're only talking a couple of months, you've probably got a good chance of your skin bouncing back, especially if this is your first. if it's been a few years already, then it's possible the loose skin won't go away on its own, and unfortunately exercise doesn't help skin, just muscle.