Once you're free to explore the world, set the difficulty to easy then enter a circuit race(there is one called tornado near where you start). Here, your car performance is around 125 while the opposition is at 105-110 so you are much quicker.
Simply trail the pack in last place for the first 2 laps, then overtake everyone and win the race. You should win easily on easy with the performance difference.
EDIT - a few people have pointed out they did it online(which will work), however I did do it offline