I had 4600 km after all races and some random driving on highways finding roads and testing cars. There is a nice trick you can drive on water (see video) and gain miles. I recommend using a Bugatti Veyron (Tuned), its easy to get on the pier and goes really fast. Use a rubberband or tape on the right trigger and leave your controller. I've gone 220 km away from island with no problems, comment below how far you've gone without problems. It took me under an hour to get the last 400 I needed driving at 429kph. * Glitch does not work with dlc Bugatti Sang Bleu. If for some reason the glitch don't work I'd recommend driving on the highways or long straight country roads, or work towards other achievements in clubs etc.
Check Miles:
In your house -> Info -> MyTDuLife -> Edit -> One down, Change -> Overall progression.
Video credit: fundreamer