The formula with 6 substituted
miles = 6 ÷ 1.609344 = 3.728
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 km takes about 72 minutes. At an easy running pace of 10 km/h it takes about 36. In metres it is 6,000.
What 6 kilometres means in practice
6 km is 3.73 miles, fifteen laps of a track, and a typical mid-week training run for someone preparing for a 10K. About 72 minutes as a walk.