For this achievement alone, you'll set 6-8 hours to complete, as you'll race ALL 111 (as of this writing) RACES in the game. There's two extra tracks in Brazil Cup.
As for which car you should use, go with one you are comfortable with, you can't change cars after starting the endurance. I chose Nano because of its very high handling and high fuel stats, even though it's one of the slowest cars in the game (I'm quite aggressive when overtaking, so with this car I hit the sides less).
Oh, and before I forget, if you don't have that much time (or want to take a break, you really should anyway), don't do it in one sitting. When you're done for the time being, wait until you see the next track and force quit the game (everyone's favourite Alt+F4). When you come back, you'll start where you left off.