This information can be found on any map that includes the latitude and longitude lines.
Alternately, a number of online resources provide the formula for computing the land distances between two points on the earth, given their latitude and longitude.
Of course, both of these methods of computation require complicated computations and do not contribute to our understanding of the program design. So, the next possibility is to measure the distance as the distance between two points that we display when we represent the cities as dots in our graphics window.
Finally, if your program does not include a visual representation of the map, the following approximation will give you realistic distances between any two cities in the 48 US states: