One of my favorite bloggers suggests (half tongue-in-cheek, I think, and quoting another) the best athlete in the world is approximately twice as good as the average Joe or Jane.
He acknowledges it fails, and in fact, his examples show it fails as much or more than it holds. Think of golf: if Tiger can shoot 72 on a given course, I'm pretty sure I can shoot 144.
I think the comparison works quite well in running. For the marathon, 2:04 maps to 4:08, probably a good "average Joe" number. For the 10K, 27:01 (on road) maps to 54:02; same. 5k, 12:37 maps to 25:14.