This is pretty self explanatory. You need to drive 500 miles total over all of your saved game. The best way to do this is just grab a fast car and go have fun in the streets. If you want to get this early in a new game it is best to drive to discover every location on the map. There is an upper and lower part to the city of Stilwater, with lots of open roads and long highways. Just take a cruise down these streets and this achievement will be yours.