The formula with 4.5 substituted
miles = 4.5 ÷ 1.609344 = 2.796
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, 4.5 km takes about 54 minutes. At an easy running pace of 10 km/h it takes about 27. In metres it is 4,500.
What 4.5 kilometres means in practice
4.5 km is 2.80 miles, a long walk of about 54 minutes or an easy 25 minute run. It is also close to the distance many European cities consider the practical limit of a walking commute.