How many kilometers long is an official marathon course? (1 km ≈ 0.6 mi)

Answer

42.2 km

The oddly precise distance comes from the 1908 London Olympics, where the course was stretched so the race could start at Windsor Castle and finish in front of the royal box. That one-off royal convenience became the official standard in 1921.

Source: World Athletics (distance standardized by the IAAF in May 1921)