Most know the story of how the marathon originated. Pheidippides, a Greek messenger, rushed from Marathon, Greece, to Athens in circa 490 B.C. to deliver news of a military victory before collapsing and dying. It is extremely unlikely that the exact distance that he had run was 26 miles and 385 yards, the distance of the marathon today. His route has been estimated at around 25-26 miles.
The first modern marathon took place at the 1896 Olympics, mimicking that Marathon-Athens route. The exact length of this rate is not known but is estimated at 24-25 miles. The initial Boston Marathon was run the following year. Its length was 24-1/2 miles, and it remained at this distance through 1923.
However, the distance of the Olympic marathon fluctuated through the following years. In 1900, it was measured at a little more than 25 miles while the 1904 course was just under 25 miles long.
The 1908 Olympic marathon, which took in London, is where the famous distance used today was first utilized. Although the details weren’t definitively recorded, some believe that its start line was by Windsor Castle’s royal nursery so that the children there could watch the race commence, but others say that reasoning is a myth.
Regardless, the general consensus is that, after the 26 miles were traversed to the Olympic Stadium, the extra 385 yards were added on to allow the runners to circle the track and finish in front of the royal box. Meanwhile, some sources say that the starting line being pulled back to the nursery was what accounted for those 385 yards.
However, that distance did not stick as the Boston Marathon continued to be run at 24-1/2 miles while the 1912 Olympic marathon was just under 25 miles and the 1920 race was more than 26-1/2 miles.
But, in 1921, the International Association of Athletic Federations officially adopted the length used at the 1908 Games as the standard marathon distance. The Boston Marathon got on board with that length in 1924 while the Olympics did so later that year. It’s unclear why that particular measurement was chosen.