The formula with 2.9 substituted
miles = 2.9 ÷ 1.609344 = 1.802
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, 2.9 km takes about 35 minutes. At an easy running pace of 10 km/h it takes about 17. In metres it is 2,900.
What 2.9 kilometres means in practice
2.9 km is just short of 3 km and comes out at 1.80 miles. It is the sort of distance a wrist tracker reports for a walk that felt like two miles but took a shortcut on the way back.