But for the 1908 London Olympics, it ended up being 26.2 miles.
The finishing line was left unchanged, but in order that the spectators, including Queen Alexandra, could have the best view of the final yards, the direction of running was changed to "right-hand inside" (i.e. clockwise) and the total distance became 26 miles 385 yards (42.195 km).
Why the history lesson?
Gabby and I recently went to Windsor, and of course, as a runner (and especially for Gabby as a marathon finisher), it wouldn't be complete without a visit to the Long Walk. For those of you who run marathons, as you run past the 26 mile marker and curse as you run another 0.2 miles, you now know it was all so that the royal family could get the best view.