The formula with 6.4 substituted
miles = 6.4 ÷ 1.609344 = 3.977
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, 6.4 km takes about 77 minutes. At an easy running pace of 10 km/h it takes about 38. In metres it is 6,400.
What 6.4 kilometres means in practice
6.4 km is four miles, or near enough: four miles is 6.437 km. It is sixteen laps of a track and a common distance in military and firefighter fitness standards written in miles.