The formula with 4.2 substituted
miles = 4.2 ÷ 1.609344 = 2.610
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, 4.2 km takes about 50 minutes. At an easy running pace of 10 km/h it takes about 25. In metres it is 4,200.
What 4.2 kilometres means in practice
4.2 km is 2.61 miles and, to many people, a marathon with the decimal in the wrong place: the marathon is 42.2 km. As a training run it is a short easy day, about 25 minutes at a relaxed pace.