I've long held the opinion that doing the longer stuff is more fun than the shorter races. Finishing the events is where the satisfaction comes in. After all, if you are like me you will never be able to do a sub 38 10k, but can you push yourself to keep going to do 30, 40, 50, 60 miles. The answer is YES. It is all mental when it comes to middle of the night and you're cold and wet and tired. But when the sun rises and you hear the first birds, the feeling of exhilaration is amazing.
Start slow, walk the ups, jog the downs and flats, eat and drink plenty. Dont stay too long at the CP's. And when you feel its hurting, think of your loved ones and smile.
Good luck to you.