Please note as per the achievement decription Time Attack does not count toward this achievement. Nor menu time, this achievement is only calculated when you're actually competing in any mode but time attack.
Set up the following:
Offline>Custom Race>Banana Bay>Poseimon
Pick Random Juicer or Juicer>5 laps
Then pick Easy and put 10 Vehicles in Race
Now pick any racer and confirm, then when the race begins just sit there. The AI should take about 9 minutes (anywhere between 9m10s and 9mins25s) before the race automatically ends.
After you've unlocked:
66 races should net you the 20 hour achievement based on 9m15sec average race time
Just pick Restart each time the race ends and the achievement pops at the end of the completed race once another 10 hours racing time has been completed.
Obviously another option is just to play regularly, depending on the enjoyment you get from this game. I do wish time attack counted toward the time, I don't understand that decision as if you are going for the other achievements you'll spend a good few hours there.