The formula with 3.8 substituted
miles = 3.8 ÷ 1.609344 = 2.361
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, 3.8 km takes about 46 minutes. At an easy running pace of 10 km/h it takes about 23. In metres it is 3,800.
What 3.8 kilometres means in practice
3.8 km is looked up mainly by triathletes: the full-distance swim is 3.86 km, or 2.4 miles, and 3.8 km is how it is usually written. As a walk it is 2.36 miles and about 45 minutes.