The formula with 25 substituted
miles = 25 ÷ 1.609344 = 15.534
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, 25 km takes about 300 minutes. At an easy running pace of 10 km/h it takes about 150. In metres it is 25,000.
What 25 kilometres means in practice
25 km is 15.53 miles, a distance used in a handful of European road races and as the point in marathon training where long runs start to feel long. About 2 hours 20 at a steady running pace.