The formula with 1 substituted
miles = 1 ÷ 1.609344 = 0.621
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 km takes about 12 minutes. At an easy running pace of 10 km/h it takes about 6. In metres it is 1,000.
What 1 kilometres means in practice
One kilometre is the metric baseline: 1,000 metres, about 12 minutes at a walking pace and a little under 5 minutes for a steady runner. It is a touch under two thirds of a mile, which is why the rule of thumb for quick conversions is to multiply kilometres by 0.6 and add a little.