Why are marathons 42.195 km long?

Marathons are 42.195 kilometers long due to the distance run in the 1908 London Olympics. Originally, the marathon was set at 26 miles, but to accommodate the royal family's viewing, the course was extended to 26 miles and 385 yards, which is approximately 42.195 kilometers. This distance was standardized in 1921 by the International Association of Athletics Federations (IAAF) and has been used in marathons worldwide since then.