The formula with 6.5 substituted
miles = 6.5 ÷ 1.609344 = 4.039
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, 6.5 km takes about 78 minutes. At an easy running pace of 10 km/h it takes about 39. In metres it is 6,500.
What 6.5 kilometres means in practice
6.5 km is 4.04 miles, and it is looked up so often because it sits at the four mile mark that many beginner plans use as the step between a 5K and a 10K. Sixteen and a quarter laps of a track; about 40 minutes at a steady jog.