The marathon distance was standardised by British royalty

The Olympic marathon distance was not fixed for the first several Games; it varied between 40 and 42 kilometres. At the 1908 London Olympics, the course was planned to start at Windsor Castle so the royal children could watch from the nursery, and to finish in front of the royal box — a distance of exactly 26 miles 385 yards (42.195 km). The distance was officially standardised at this length from 1921. Every marathon runner today owes their precise target to an ad hoc decision made to accommodate royal viewing arrangements.

Source: Martin & Gynn, The Olympic Marathon (2000)