The formula with 5.5 substituted
miles = 5.5 ÷ 1.609344 = 3.418
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, 5.5 km takes about 66 minutes. At an easy running pace of 10 km/h it takes about 33. In metres it is 5,500.
What 5.5 kilometres means in practice
5.5 km is 3.42 miles, a 5K with a warm-down tacked on, and a typical figure for a lunchtime run that starts and ends at the office door rather than at the start line. About 33 minutes at an easy pace.