Read the excerpt The Ancient City.

The adult games started with the stadion, the 184-metre (200-yard) sprint. (This was the most prestigious of all the events at Olympia: the winner gave his name to the Olympiad, the four-year dating system.) This was followed by the long distance race of twenty or twenty-four lengths of the track (3680 metres/4024 yards or 4416 metres/4829 yards).
The third track event, the hippios, was a middle distance race of six lengths, a little over 1000 metres (1093 yards). It was probably called the hippios, horse race, because the distance was the same as that run by horses.

The author includes details comparing races to show that

races were of different lengths.
all races had the same prize.
Greeks favored long races.
people ran in races against horses.