I just purchased my first road bike: Specialized Roubaix. I've been riding it a week and after about ten miles into every ride, my lower back becomes unbearably sore and I'm forced to quit by at best mile fourteen.

I haven't changed anything on the bike yet and have not upgraded to clipless pedals. The shop set my seat at the correct height, but it's just the standard seat that came with the bike.

Is there anything I can do to increase my comfort? I'm starting to train for a few century rides a few months away, but if I'm already having this much of a problem with my back, this could be a pretty large issue.

Thoughts, comments, suggestions? All help is very much appreciated!