The formula with 21.1 substituted
miles = 21.1 ÷ 1.609344 = 13.111
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, 21.1 km takes about 253 minutes. At an easy running pace of 10 km/h it takes about 127. In metres it is 21,100.
What 21.1 kilometres means in practice
21.1 km is the half marathon, exactly 21.0975 km or 13.109 miles, half the marathon distance defined in 1921. Most recreational runners finish between 1 hour 45 and 2 hours 30.