Dead easy, just bimble along at the back, you don't even have to race or complete the race distance. Just stay out of the way of the real racers as you don't want to spook them with a random ghost car. Do the short A races, you'll have it done in no time at all and as a bonus, your DR will have massively dropped so when you decide to race for real you will get poles, wins and fastest laps on your way back up to your regular DR! Win-win situation!