The formula with 20 substituted
miles = 20 ÷ 1.609344 = 12.427
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, 20 km takes about 240 minutes. At an easy running pace of 10 km/h it takes about 120. In metres it is 20,000.
What 20 kilometres means in practice
20 km is 12.43 miles, a distance that appears in race walking (the 20 km walk is an Olympic event) and as a round-number long run. It is 1.1 km short of a half marathon.