"Those who say it cannot be done should not interrupt those doing it."

I went for another long run yesterday. 13 miles in 2 hours, same time as the first 13 miles of my run Christmas Eve. I thought that it would start getting easier, but it's not. Still getting muscle and knee pain, mostly on my right side. Not sure if I'm doing something wrong but if anyone has any suggestions I'ld be glad to hear them.
It's funny running that distance in a small town, I practically have to run around the WHOLE town in order to not retrace my steps.
Oh, and my GPS watch is AWESOME!