The marathon was NOT an event of the ancient Olympic games. The marathon is a modern event that was first introduced in the Modern Olympic Games of 1896 in Athens, a race from Marathon northeast of Athens to the Olympic Stadium, a distance of 40 kilometers.
The race commemorates the run of Pheidippides, an ancient "day-runner" who carried the news of the Persian landing at Marathon of 490 B.C. to Sparta (a distance of 149 miles) in order to enlist help for the battle. According to the fifth century B.C.ancient Greek historian Herodotus, Pheidippides delivered the news to the Spartans the next day.
The distance of the modern marathon was standardized as 26 miles 385 yards or 42.195 km. in 1908 when the Olympic Games were held in London. The distance was the exact measurement between Windsor Castle, the start of the race, and the finish line inside White City Stadium.