You may unlock this naturally while going for all of the other cup titles, but if not you can start up a random single player race and race towards the finish line (auto-steer and auto-acceleration optional). Towards the end of the race, you can sit there before the finish line and walk away from your controller. Every once in awhile you may have to press a button or two to prevent the controller from disconnecting, then when you're ready to bank the time earned, simply finish the race.
This achievement only counts and unlocks following the conclusion of a race in any mode.