The length of marathon races was set by?

The marathon may have ancient roots, but the foot race’s official length of 26.2 miles wasn’t established until the 20th century. As the story goes, Queen Alexandra requested that the race start on the lawn of Windsor Castle (so the littlest royals could watch from the window of their nursery) and finish in front of the royal box at the Olympic stadium—a distance that happened to be 26.2 miles (26 miles and 385 yards). The random boost in mileage ending up sticking, and in 1921 the length for a marathon was formally standardized at 26.2 miles (42.195 kilometers).