as a follow up to the post below, ny times has a nice little article about "not really running." basically, the little mind tricks runners/athletes play on themselves to keep going.
the article is great with little stories like one about a pole vaulter who cleared the pole every time with about a foot to spare, up to a certain point. but if his coaches moved the bar up even an inch, the vaulter would hit it every time. so one day, "when the vaulter was not looking, his teammates raised the bar a good six inches. the man vaulted over it, again with a foot to spare."
or one about these tibetan monks who reportedly ran 300 miles in 30 hours, an average pace of six minutes a mile. "their mental trick was to fixate on a distant object, like a mountain peak, and put their breathing in synchrony with their locomotion. every time a foot hit the ground they would also repeat 'down'."
i may not be a tibetan monk or pole vaulter, but the same tricks apply as i plod along on my runs. i tend to break it down into crazy math formulas and fractions that generally get most ridiculous and complex just before my intended halfway mark. just double what i've already run and then add a mile and then another half of that and we're done. or just 9/16s further, plus another 2 miles and then half of that for a cool down. depending on the day it gets worse, but we all have our tricks.