Use a Ruler, or Curvimeter, or Even Google Maps Distance Calculator.
If you want to calculate the real world distance of a route, you need first to measure the equivalent distance on the map. This step is simple if the route is a straight line between two points. All you need to do is to measure the distance with a ruler. If you don't have a ruler, bear in mind that usually there are inscribed rulers in the base plates of orienteering compasses. Another way is to use drawn grid lines across the maps as measuring tool. The grid lines form an array of identical squares with sides of a specific length. After figuring out the dimensions of one square, the grid effectively becomes a ruler. For more complex routes, divide your route into straight-line segments and measure the distance of each segment one by one. Simply add the length of all segments to calculate the total length of the route.