Why are very long races known as marathon events?

The highlight of long distance racing in the Olympics is the marathon.It takes its name from a legendary run made by a Greek soldier in 490 BC. He raced from the plain of Marathon to Athens to announce the news of great victory over an army of invading Persians.

In 1896,at the first modern Olympics in Athens,it was decided to stage a long road-race.Most of 16 runners in the first marathon were Greek,but they were untrained with exhaustion. The winner,a Greek called Spiros Louis,was  finally joined by members of the Greek royal family who jogged alongside him on the last lap to the finish line. His victory was a sensation.
Since 1908,the marathon has been fixed at 42.2 kilometres,a distance regularly run in about two and a half hours nowadays.