For those who want to do this as fast as possible, I suggest finding long stretches of road (like the split highway) to do this on. As long as you are using an S1/S2/X class car and do not challenge off road cars, you can challenge someone, race, then after the race spam A, turn around, and within 30 seconds or so (a minute at the most), another driavatar will end up in front of you. Then you blast down the highway again, then keep repeating. I did this in 30-40 minutes this way.