The formula with 6.8 substituted
miles = 6.8 ÷ 1.609344 = 4.225
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, 6.8 km takes about 82 minutes. At an easy running pace of 10 km/h it takes about 41. In metres it is 6,800.
What 6.8 kilometres means in practice
6.8 km comes out at 4.23 miles, seventeen laps of a track. It is a common length for a river or seafront out-and-back where the turnaround is a landmark rather than a measured point.