This is 10 hours of game time, which means you will need 10 hours worth of race, battle, stunt, drift, and/or trick activity.
The trick to this is to simply just sit in front of Kinect while in a race. The car shouldn't move (as long as you don't move too much) and you will rack up a lot of time. I personally just sat and did work.
Another great solution (thanks to Scrapdaddy) is to have a cardboard cut out or construct something Kinect will mistaken for a person. Once you have done that, use your arms to start the race then move out of the way without having the Kinect detect you. This should allow you to go do a lot of other things like groceries, movies, etc. This did work for me but I could not use the technique since I have two kittens. They both ruined it multiple times.
The level you pick is entirely up to you. If you can't keep yourself from driving then the last level is the best since it is decked out with several turns.