According to the Wikipedia:
"The Spartathlon aims to trace footsteps of Pheidippides, an Athenian messenger sent to Sparta in 490 BC to seek help against the Persians in the Battle of Marathon. Pheidippides, according to an account by Greek historian Herodotus in The Persian Wars, arrived in Sparta the day after he departed. Herodotus wrote: "On the occasion of which we speak when Pheidippides was sent by the Athenian generals, and, according to his own account, saw Pan on his journey, he reached Sparta on the very next day after quitting the city of Athens.
Based on this account, British RAF Wing Commander John Foden and four other RAF officers travelled to Greece in 1982 on an official expedition to test whether it was possible to cover the nearly 250 kilometres in a day and a half. Three runners were successful in completing the distance: John Foden (37:37), John Scholtens (34:30) and John McCarthy in (39:00). In the following year a team of enthusiastic supporters (British, Greek and other nationalities) based on the British Hellenic Chamber of Commerce in Athens and led by Philhellene Michael Callaghan organised the running of the first Open International Spartathlon Race. The event was run under the auspices of SEGAS, the Hellenic Amateur Athletics Association.
The race starts at 7:00 am, usually on the last Friday each September, at the foot of the Acropolis. It runs out of Athens toward the coast and runs along the coast towards Corinth via Elefsis, Megara, and Kineta. The route reaches the Corinth Canal at 78.5 kilometres and the runners hit the first of six major check points at 81 kilometres.
After Corinth, the race heads toward Ancient Corinth, Nemea, Lyrkia and at 159 kilometres, reaches the top of Mount Parthenio. From there, it continues south toward Nestani and Tegea, before reaching the main Sparta highway just before the 200 kilometer mark.
Runners must pass through 75 checkpoints along the way and each checkpoint has a cut-off time. Runners outside the cut-off may be pulled out of the race although tardiness in the first half of the race is generally tolerated. This tolerance begins to fade after sunset and in the last third of the race, organisers may pull out runners who are either outside the time limit or who display extreme fatigue."