One of the most iconic races of the Ultra season is the 246km Spartathlon held annually in Greece. The Spartathlon traces the footsteps of Pheidippides, the legendary 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 36 hours later, hence the 36 hour cut off for the race.