Race editor races in Cruise Do Not count for this achievement.
Fastest method i found is a private Basewar
to bring up the Modes menu and select:
Basewar Capture The Flag - River RushPoint Limit
Now once the game starts the player not
going for the win steals the other player's flag, then the player who is
going for the win steals back their flag (By touching cars) and takes it back to their base point, Race complete. You're then taken back to the lobby where you can ready up and repeat the process.
With the beginning cars the 'race' takes around 1 minute.