What is the nature of suffering when running?

Someone posted previously (I think KSH) about the difference between hurting from injury and hurting from pushing your body to the limit. When I am swimming or cycling, I know when I'm tapped out, muscles hurting, quads completely empty, all-over aching but in a feel-good way.

However, when I run it hurts in an all-different way, usually when I'm past my "comfort zone" of 5-6 miles. Sharp jarring pain in my lower back, shooting pains up the backs of my thighs. I usually stop because of these pains, not because my aerobic capacity has been reached. There seems to be no aftereffects on the day-after, other than a slight stiffness.

Since I've never been much of a runner, I don't know if these pains are the usual "suffering" everyone goes through, or if it is symptomatic of injury/damage. I can run a 5k with no problems, but if I'd like to do the Oly distance.