The formula with 8 substituted
miles = 8 ÷ 1.609344 = 4.971
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, 8 km takes about 96 minutes. At an easy running pace of 10 km/h it takes about 48. In metres it is 8,000.
What 8 kilometres means in practice
8 km is 4.97 miles, twenty laps of a track, and the 8K is a recognised road race distance in the United States where it is often billed as roughly five miles. About an hour and a half on foot.