The formula with 50 substituted
miles = 50 ÷ 1.609344 = 31.069
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, 50 km takes about 600 minutes. At an easy running pace of 10 km/h it takes about 300. In metres it is 50,000.
What 50 kilometres means in practice
50 km is 31.07 miles and the shortest standard ultramarathon, about five miles further than a marathon. It is also the distance of the longest race-walking event ever held at the Olympics.