The formula with 3,500 substituted
miles = 3,500 ÷ 1.609344 = 2,174.799
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, 3,500 km takes about 42000 minutes. At an easy running pace of 10 km/h it takes about 21000. In metres it is 3,500,000.
What 3,500 kilometres means in practice
3500 km is 2,175 miles, a distance that comes up in questions about road trips and flights: it is roughly Lisbon to Moscow by road, or a little more than the drive from New York to Denver. At 8 L/100km a car would burn 280 litres covering it.