Sorry to correct you but...
The choice of distance was somewhat arbitrary. The first modern Olympics in 1896 had a marathon distance of 40.8 km. The marathon at the 1908 Olympic Games in London began at Windsor Castle and was extended by 385 yards from its set 26 miles so that the runners would cross the finish line in front of the Royal family's viewing box in the Olympic arena, so the length became 42.195 km (26 miles 385 yards). For the next Olympics in 1912, the length was changed to 40.2 km and changed again to 42.75 km for the 1920 Olympics. Of the first 7 Olympic games, there were 6 different marathon distances between 40 and 42.75 km (40 km being used twice). A fixed distance of 42.195 km was adopted in 1921 by the International Amateur Athletic Federation (IAAF) as the official marathon distance - Rule 240 of their Competition Rules.
(From Wikipedia)