The formula with 2.8 substituted
miles = 2.8 ÷ 1.609344 = 1.740
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, 2.8 km takes about 34 minutes. At an easy running pace of 10 km/h it takes about 17. In metres it is 2,800.
What 2.8 kilometres means in practice
At 2.8 km you are at 1.74 miles, seven laps of a standard track exactly. It is a common commuter cycling distance in flat cities, taking under ten minutes on a bike.