The formula with 2.6 substituted
miles = 2.6 ÷ 1.609344 = 1.616
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, 2.6 km takes about 31 minutes. At an easy running pace of 10 km/h it takes about 16. In metres it is 2,600.
What 2.6 kilometres means in practice
A 2.6 km run is a little over 1.6 miles, and it is a popular value because it is where a 1.5 mile course with a cool-down lands on a watch. Around 13 minutes at an easy jogging pace, or 31 minutes walking.