What distance did the 1908 London Olympic marathon establish as the modern standard?

The 1908 London Olympic marathon established the modern standard of 26 miles 385 yards.

The race was planned to begin at Windsor Castle so that the royal family’s children could watch the start. Its finish was placed in front of the royal box at White City Stadium, producing a total distance of 26 miles and 385 yards.

Earlier Olympic and marathon races used varying distances, because the event had not yet been standardized. The 1908 course became the model for later races, and the distance was formally adopted internationally in 1921.

In metric terms, 26 miles 385 yards equals 42.195 kilometres. Marathon organizers often round this to 42.2 kilometres, but certified courses must use the exact official distance.