[Marathon ](http://en.wikipedia.org/wiki/Marathon_(sports\))is exactly 42.195km. Why? Why isn't it 40km or 30km or 50km? and where did this word marathon came from?
The name marathon comes from the legend of Pheidippides, a Greek soldier, who was sent from the town of Marathon to Athens (distance is roughly 40km) to announce that the Persians had been defeated in the Battle of Marathon.
In the modern Olympics, at first the exact distance of 42.195km was not fixed until 1908.