The formula with 3.5 substituted
miles = 3.5 ÷ 1.609344 = 2.175
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, 3.5 km takes about 42 minutes. At an easy running pace of 10 km/h it takes about 21. In metres it is 3,500.
What 3.5 kilometres means in practice
3.5 km is 2.17 miles, a common leg on urban walking routes and the length of many riverside promenades. Walking it takes about 42 minutes; it is 70% of a 5K.