Race editor races in Cruise Do Not count for this achievement.Fastest method i found is a private
Basewar match.
Press
then
to bring up the Modes menu and select:
Xbox Live
Player Match
Create Match
Basewar Capture The Flag - River Rush
Point Limit - 1.
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.