The formula with 2.5 substituted
miles = 2.5 ÷ 1.609344 = 1.553
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, 2.5 km takes about 30 minutes. At an easy running pace of 10 km/h it takes about 15. In metres it is 2,500.
What 2.5 kilometres means in practice
2.5 km is 1.55 miles, exactly half of a 5K, and the point at which most parkrun courses turn for home. It is also a common minimum for a lunchtime walk that counts as real exercise: about 30 minutes.