After you have accumulated all the miles required to unlock all the cars, you will likely be left with several hundred miles to fill, and not much desire to do so. However, by using elastic bands, and the Pike Peaks hill climb track in tier 8, you can do it while you sleep (provided you have a wired controller). It took me about 11 hours to do just over 450 miles this way, without any of my effort.
On that track, about 30% of the way up, there is a hairpin right turn, with a gate, and railing. If you crash through the gate, you will fly through the air and then respawn. Simply by holding down RT, this process will continually repeat. Make sure that you have damaged your front left wheel so that the car tends to drift slightly to the left to ensure that you do not make any progress.
You can then just leave your Xbox to do its work. If you lack a wired controller, just watch TV and fiddle with the right analogue stick every now and again.
STAFF EDIT: Unavailable video removed.