The formula with 12.5 substituted
miles = 12.5 ÷ 1.609344 = 7.767
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, 12.5 km takes about 150 minutes. At an easy running pace of 10 km/h it takes about 75. In metres it is 12,500.
What 12.5 kilometres means in practice
12.5 km is 7.77 miles, and the question 'how many miles is 12.5 kilometers' is a common one because 12.5 is a round fraction of 100 and turns up in exam questions and in fitness plans that build in 2.5 km steps. Just over 31 laps of a track.