The New York Times recently ran a great article on running and dissociation. I know I have used the technique at times, though mostly unconsciously.
According to the article, performance and endurance have at least as much, if not more, to do with the brain as it does the body. By "dissociating," or focusing on something other than what your are doing, you can override your brain's reluctance to push past pain and what you think your limits are.
Give the article a read, and on your next long run try it out and see for yourself. I know I am going to work on this technique more.