The formula with 1.8 substituted
miles = 1.8 ÷ 1.609344 = 1.118
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, 1.8 km takes about 22 minutes. At an easy running pace of 10 km/h it takes about 11. In metres it is 1,800.
What 1.8 kilometres means in practice
1.8 km is a common lap length for canal towpaths and reservoir circuits, and it is just over 1.1 miles. Two laps make a comfortable 2.2 mile walk; four laps are a 4.5 mile run.