When the first modern Olympic marathon was held in 1896 in Athens, the course was about 25 miles long, roughly matching the legendary run from the town of Marathon to Athens. The distance varied slightly at each subsequent Olympics, usually falling between 24 and 25 miles, depending on the route.

That changed in 1908, when the London Games planned the start at Windsor Castle so the royal children could watch the runners set off. King Edward VII and Queen Alexandra wanted the race to finish directly in front of the Royal Box at White City Stadium, so the course was extended to 26 miles and 385 yards.

The distance felt right to organizers, and in 1921 it became the official standard for all marathons.