Bluetree, you could be hurting because, as you said, you're out of your comfort zone, and pushing your body to go farther than it's accustomed to. I've always thought that runners have a high tolerance for pain because running always hurts in some manner, but you learn what's a "good" hurt and what's a "bad" hurt after a while. If you're not in pain the next day, you're probably fine, but if you have pain other than muscle soreness or stiffness the next day, you should back off the distance. I will say that I have had the sharp, shooting pains up the backs of my legs, and that was not a good thing -- turns out I had pinched nerves in my back. So, watch the shooting pains, since they usually are not a "good" hurt, and build your mileage VERY slowly. By the way, are your shoes in good condition and appropriate for your feet? I've had more problems from wearing the wrong shoes than anything else.

KB