Easiest and most fun way to get kilometres driven (and stunt points!):
Go to the international airport, call in a car from black market, wait at the beginning of one of the big runways for a 747 to come, tether your car to the plane's nose (might take a couple of tries to get them to stick together), and get in your car.
Get ready for the ride of your life and kilometres driven! Also give you stunt points which contribute towards Stunt Driver achievement.
(Just don't hit B to jump out into stunt position on your car, the tether to the plane will break for some reason)
EDIT: Added a video I just recorded!