Adding further
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Really on race day you are running to the end. Let me explain. When training you can always stop and try again later or just bag the run. Your mind has learned little triggers that will cause pause and start you to think, I just cant go any longer. Its triggered because the brain is trying to preserve itself and food supplies. Now since you have SUCESSFULLY run 20 miles you have developed a few games of your own that could be called work arounds. These are those simple little cues that keep you going to the end of your long runs. It may be just run to the next light post, keep in touch with the runner in front of me, dedicating a mile to someone who supports you. (note if you have not thought about this do so by just reviewing what thoughts or actions got you to the end of the tough runs.
As posted above you do have the crowds which will get you through the last three miles. These crowds will also carry you through those little tough areas that you will inevitably experience earlier in the race. Crowds may only be idle talk with the runner next to you but it is something that keeps you focused on whats ahead, not whats happening.
Finally, know that you must get to the end will carry you through some of the tuff areas. Its a simple motivator but a powerful one. Note that you will need to break the end up at times to the next pole, next aid station or next crack in the pavement. As you have experienced with your 20 mile runs, you will start to feel the feelings you described. Trust your training; trust you hydration; trust your nutrition; trust your will to move forward.
You are ready to do this. You have overcome the toughest part of the event training. Use race day as a celebration and go out and have fun! Its OK to feel uneasy at 4:30 on January 10 as most runners in the gathering area are feeling it to some degree
they are lying if they tell you they have no uneasiness. Talk with you new friends, enjoy the festive atmosphere pre-race, have fun on the march to the corrals, enjoy the Star Spangled Banner and fireworks. You are ready and will soon be marathoners.