Have you ever done a marathon? It is a 26 mile long race.
I have not run a marathon.  My wife, however, enjoys 1/2 marathons.
Impressive. The marathon is run to commemorate a Greek soldier, Pheidippides. He was a messenger from the Battle of Marathon to Athens.
How far did Pheidippides run to carry his message?
Right around the 26 miles distance. There are around 800 marathons throughout the world every year. Tens of thousands of people compete in them. That is so crazy to me. 
I had no idea there were so many.  Which marathon is the biggest one?
Most likely the Olymipcs. The marathon has been in the Olympics since 1896.