The Marathon Distance Was Fixed Arbitrarily in 1908

The marathon's standard distance of 26.2 miles (42.195 km) was set at the 1908 London Olympics — not for athletic reasons, but to accommodate the British royal family. The race was extended so it could start beneath the nursery window at Windsor Castle and finish in front of the royal box. The International Association of Athletics Federations standardised this distance in 1921.

Source: The Marathon: The History of the Greatest Race on Earth — various athletics historians