Why is the marathon 26.2 miles?
The exact distance of 26 miles 385 yards was standardized in 1921.
The first modern Olympic marathon in 1896 covered roughly 40 km, and later Games used various lengths. In 1908 the London organizers set a course from Windsor Castle to White City Stadium, finishing in front of the royal box. The distance measured 26 miles 385 yards, or 42.195 km. A popular account says the start was placed so the royal children could watch from the castle nursery windows, though details differ between tellings.
That race became famous for the finish. The Italian Dorando Pietri entered the stadium first but collapsed repeatedly, was helped across the line by officials, and was disqualified for the assistance; the American Johnny Hayes was declared winner. Pietri's plight drew a huge public reaction and made the distance well known.
Marathon lengths were not uniform until later. The International Amateur Athletic Federation adopted 42.195 km as the official standard in 1921, and the 1924 Paris Olympics were the first to use it. The legend that inspired the race, a Greek runner carrying news from Marathon to Athens in 490 BCE, is historically doubtful, and the actual distance from Marathon to Athens is around 40 km. The 26.2 figure therefore comes from the English royal route, not from antiquity.