The formula with 30 substituted
miles = 30 ÷ 1.609344 = 18.641
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, 30 km takes about 360 minutes. At an easy running pace of 10 km/h it takes about 180. In metres it is 30,000.
What 30 kilometres means in practice
30 km is 18.64 miles and the traditional peak long run in marathon training, on the theory that the last 12 km on race day will take care of themselves. A 30 km race is also common in Europe.