The formula with 42.2 substituted
miles = 42.2 ÷ 1.609344 = 26.222
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Kilometres to Miles Converter explains where each comes from and gives a longer table.
At a walking pace of 5 km/h, 42.2 km takes about 506 minutes. At an easy running pace of 10 km/h it takes about 253. In metres it is 42,200.
What 42.2 kilometres means in practice
42.2 km is the marathon, exactly 42.195 km or 26.219 miles, a distance fixed at the 1908 London Olympics and made standard in 1921. It is 105.5 laps of a running track.