The formula with 8.5 substituted
miles = 8.5 ÷ 1.609344 = 5.282
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, 8.5 km takes about 102 minutes. At an easy running pace of 10 km/h it takes about 51. In metres it is 8,500.
What 8.5 kilometres means in practice
8.5 km is 5.28 miles, a touch over five, and a typical evening run for someone in the middle of a 10K plan. It takes a recreational runner 45 to 55 minutes and a walker about an hour and three quarters.