The formula with 3.6 substituted
miles = 3.6 ÷ 1.609344 = 2.237
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, 3.6 km takes about 43 minutes. At an easy running pace of 10 km/h it takes about 22. In metres it is 3,600.
What 3.6 kilometres means in practice
3.6 km comes out at 2.24 miles, nine laps of a running track. It is a frequent figure for a short cycle commute or a long dog walk.