Your mental approach to a race is one of the most important parts of your build up. Before a big event you have to have a good conversation with yourself and ask yourself;
'What am I prepared to go through? How much am I prepared to hurt and suffer to achieve what I set out to? How much does it mean to me? What are my motivators? Who am I doing it for?'
You have to know the answers inside out. So when the pain and fatigue set in, you have the answers for The Devil who will sit on your shoulder and ask you over and over; 'Why the hell are you doing this - why don't you just stop?'
There often comes a time in a race where you start negotiating and bargaining with yourself. This is dangerous territory and you can't let it happen. You may tell yourself - 'well this is the longest I have ever run at least, and I am happy with that - so even if I don't finish so what?' Those sort of internal conversations are deadly to your will power, so don't have them.
Don't focus during the race on the whole distance, break it into manageable bites. The harder it gets the smaller those bites should be.
Don't waste time ever. i.e.; If you stop - you must have a damn good reason! There are times when you should rest, or stop but keep the pressure on yourself to get out again. Control points or check points can be lethal mentally. There is a saying "beware the chair" meaning it will entice you and if you let go too long you won't get back out.