The following is some helpful advice as posted by John M. in the trail forums at Runners World Online. I like to re-read it occasionally as a reminder to myself. I hope you find it helpful as well.
Some odds and ends to get you through the day, the distance, whatever...
Believe in yourself--nothing else will get you to the finish line.
Decide before you start what will stop you--if that doesn't happen, you continue.
Are you racing or running? Time goal (sub-24, big buckle, age group) or running to finish. Don't let the initial goal be etched in stone. Something may go wrong out there--adjustments will need to be made. Make them and keep going.
Run your plan. Stay within your realm. Don't feel bad if someone passes you. Don't chortle with glee if you pass someone. Keep a sense of what your are about. Keep pressing on, maybe it is one of those good days when you pick it up and keep on picking it up.
Have faith in walking. Walk when you need to or when you want to, but walk with purpose...no trudging...no survival shuffle...keep a good mindset and walk with a purpose.
Be sure your crew (if you have one, a crew is not a necessity) understands that you might go through a transition from nice person to "not so nice" person. Have a talk with them about the need to kick your butt back out on the course. Sympathy may exist, but not to the extent of shortchanging the runner.
Problems. Is it a problem or just an inconvenience? Decide which. Find a solution for the problem. Block out the inconvenience.
Food. Stick with the safest food there is at the aid stations. Use as much of your own stuff as you can, but don't be inflexible about things not being just perfect. Be flexible as you go.
Equipment. If some equipment change comes into your head--is it a need or a want. If it is a need, solve it at the next crew or drop bag point. If it is a want and can't be fixed fairly easily, drop the thought--keep moving.
Throwing up, vomiting, coughing the cookies...it may happen even if it has never happened before. It is not fatal. It is an inconvenience. You might need more water between the point it happens and the next aid station (it can dehydrate). Drink more. Stay at the next aid station long enough to drink and eat more. Your body is now low on fuel and water. You must pay attention to eating more. You can restore the liquids fairly quickly, but you must eat every chance you get. Try not to throw up on anyone .
Don't stop. Keep moving. Low points will come, continued movement will bring you back around. Don't sit in those chairs unless you really need to--you will not really need to until somewhere past 80 miles.
Be encouraging to others. Smiles and laughter will be helpful to others. Helping others will be helpful to yourself.
Smile and joke with the aid station folks and say thank you to the volunteers. They will help you all through the day and night and...be good to them. They are a great source of energy and inspiration donating all that time to get us through our little escapade.
No externalizing of negatives. No, "Hot out here, ain't it?" No, "This is a long hill, eh?" Just believe in yourself, all that training, all those folks you ran with throughout the winter, spring, and summer that got you so strong.
It's all there...yours for the taking.
Run gently out there.