A marathon is 26.2 miles long, or exactly 42.195 kilometers (about 26 miles 385 yards). Every official marathon, from Boston to Berlin, uses that same certified road distance.
The figure traces to the 1908 London Olympics course, and the International Amateur Athletic Federation (now World Athletics) made 42.195 km the official standard in May 1921. Before then, marathon races varied in length, often around 40 km.
Source: Wikipedia, Marathon