I've heard the 10% guideline, too, although I've also read (somewhere, forget where) that when you're at a lower mileage to begin with you can add more than 10% because it's such a small amount at that level. For example I started out at 5 and easily added 1-2 miles at a time after that. I do realize that I'm talking about closer to 50% though.

I do have a chiropractor I see regularly. I will ask him next time (today) if he sees anything off in particular, but I'm pretty sure this is a muscle strain from bad position kind of thing. My overall chronic pain I'm pretty sure stems from poor posture...something I constantly work on, but can't seem to keep corrected once I stop paying attention.