I've got an oft-told story I can relate again.
When I started riding flat, I'd been pinned to the computer (internet, gaming, stuff like that) for pretty much five, six years straight. In that time, I'd managed to balloon up to nearly 200lbs... a lot, considering my body size/type. Within four months, I was down to 165lbs. No special diet, no exercise plan, just riding. I rubbed it in hard with my brother and my boss, who were both trying to lose weight through that retarded Atkins diet. After a while, I equalized at 150lbs, and I've been maintaining that fairly easily; still no special diet, still no working out. I imagine I'd be in even better shape if I rode more often than I am right now. I'd sure as hell be better.
Keep riding, you'll hit your equilibrium point soon enough, and after a couple weeks or so you won't even be getting sore like you are now. It's a freakin' workout in and of itself.