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Beejay, it's a good question. ANd the honest answer is, I don't know.
The distance is 26 miles 385 yards. Apparently they measure it on a bike, with a measuring device called a Jones Counter.
Did you notice the peculiar little hairpin that the runners had to run at the end of this Olympics? The organisers wanted the race to start in Tiananmen square (the tanks had to move aside for the day!!) and finish at the Olympic stadium, but the course didn't fit, so when the runners got to the stadium, they had to run about 50m past it, turn around a hairpin, and then come back again.
It's a good job it wasn't like a school cross country ... all the runners would have just hopped over the barrier and gone straight into the stadium.