The formula with 2.1 substituted
miles = 2.1 ÷ 1.609344 = 1.305
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, 2.1 km takes about 25 minutes. At an easy running pace of 10 km/h it takes about 13. In metres it is 2,100.
What 2.1 kilometres means in practice
2.1 km is very close to 1.3 miles, and it appears in commuter searches because it is roughly the walk from one end of a city centre to the other in a mid-sized town. About 25 minutes on foot.