Have you ever run in a marathon? It is a race with a distance of 26.219 miles, usually on a road.
I haven't! I didn't actually know a marathon was over 26 miles even though I know a lot of people who have run in one. Is that why they have to be on a road?
They don't have to be, but they would be much harder if they weren't. The Marathon was instituted in commemoration of the fabled Battle of Marathon in Athens.
Was that before or after the olympics were created?
It was after the Olympics. In the battle a Greek soldier ran from the battlefield to report the victory.