Why stop at such an awkward number?
A marathon is 42.195 kilometres, usually rounded to 26.2 miles. The modern distance comes from the London 1908 Olympic course and was standardised in 1921.
A neat 40 km would be easier to remember. Running, apparently, kept the receipt for the extra distance.
A castle, a stadium and a finish line
The Olympic Museum traces the London route from Windsor Castle to White City Stadium, finishing in front of the Royal Box.
Its recorded length was 26 miles and 385 yards, the basis of the familiar 42.195 km. The precise-looking number has a history in miles, yards and the layout of a particular course.
It did not become the rule overnight
The next Olympic marathon, in Stockholm in 1912, covered 40.2 km. The London measurement was not immediately compulsory everywhere.
World Athletics dates the standardisation to 1921. The Olympic Museum notes that the IAAF records do not explain why that particular 1908 measurement was chosen. A tidy royal anecdote cannot fill that gap in the records.
Keep the number and the unit together
42 km is 195 metres short of a marathon. And 26.2 is the rounded distance in miles, not kilometres. Those small-looking differences belong in the answer.
For another sporting symbol with a story people often misremember, discover what the Olympic ring colours actually mean. Familiar does not always mean obvious.