The marathon's 26.2 mile distance was set to fit the British Royal Family's viewing spot

At the 1908 London Olympics, the marathon was extended from 25 miles to 26 miles 385 yards so the race could start beneath a window at Windsor Castle and finish in front of the Royal Box at the Olympic stadium. The distance was formally standardised in 1921. Before that, marathons had been different lengths at every Olympics.

Source: International Association of Athletics Federations (IAAF) historical records