I just started commuting (as well as riding to go just about anyplace). After a 10 mile ride where I did some balls to the wall pedaling, when I stopped and got off the bike my legs were like jello and I collapsed.

I am going a little easier now until I am in better shape

One thing that *might* be the problem, but its just a long shot guess. Is your saddle a bad fit, could it be cutting circulation off to your legs? I've had that happen when sitting in front of a workstation too long in a crappy chair.