The length of a Marathon is about 40 kilometers.
To celebrate the running of a slave to the city Marathon.
The length of a marathon was at first not fixed, since all that was important was that all athletes competed on the same course. The exact length of an Olympic marathon varied depending on the route established for each venue. The choice of distance was somewhat arbitrary. The first modern Olympics in 1896 had a marathon distance of 40 km. The starting point for the 1908 Olympic marathon in London was modified so that the Royal Family could have a good view and the length happened to be 42.195 km (26 miles 385 yards). For the next Olympics in 1912, the length was changed to 40.2 km and changed again to 42.75 km for the 1920 Olympics. Of the first 7 Olympic games, there were 6 different marathon distances between 40 and 42.75 km (40 km being used twice). A fixed distance of 42.195 km was adopted in 1921 by the International Amateur Athletic Federation (IAAF) as the official marathon distance.
it is 42.195km
I depends if you are running a full marathon or a half marathon but a full marathon is 26.2 miles