The formula with 15 substituted
miles = 15 ÷ 1.609344 = 9.321
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, 15 km takes about 180 minutes. At an easy running pace of 10 km/h it takes about 90. In metres it is 15,000.
What 15 kilometres means in practice
15 km is 9.32 miles, a common Sunday long run and a recognised race distance in Europe. It is a little under three quarters of a half marathon and takes most runners 75 to 100 minutes.