I find it interesting that not a single person asked you what pace you are running all of you other training runs and if they are appropriate to your conditioning?
Many people try assume that if they run 15 miles all the time at their hoped for marathon pace, then it will make it easier to run 26 at this pace. The pace may not be right for your condition based on otrher PR's. You may be running too hard on those runs as well as all of your other training runs. Thus you are not training the proper muscle fibers. Are all of your other runs besides the marathon paced long run at least 90 seconds per mile slower? This would be a good start. If you are running faster than this, you are in a constant state of glycogen depletion.