For this achievement, you have to win on 10 really long races, taking about an hour each.
There are two ways to do this: Driving yourself, hiring a driver
I actually suggest you drive these races by yourself, they are not that bad, and the feeling you get after completing one of these races is priceless.
If you play on easy, you will usually complete these races 3 or 4 laps ahead of your oponents, giving you more than enough spare time to use boxes when needed.
Make sure you keep an eye on your gas tank (bottom right corner of the screen), and your tire wear (press LB), specially if your car has racing tires, as they wear faster.
Winning these races will give you a big load of credits, enough to max a car's level.
-Make sure you have time to do the race, you don't want to quit the race in the middle of it because you have to do something else.
-Listen to your favorite music, believe me, this helps a lot.
-Try not looking at the lap counter, if you're constantly checking your laps, the race will feel endless. If you need to, cover that part of the screen.
-If you want to do more than just steering and braking, select manual transmission, this will help you practice, plus you will get extra credits.
-Don't use the boxes unless you know your tires or tank won't make it for another lap, but becareful, you don't want to get an empty tank on your last lap.
-If you have a wireless controller, make sure it has enough charge, or plug your charger to play.
-You might want to use a level 0 car, the discounts obtained from leveling it might be useful.
Hiring a driver
This is the easiest way, but you won't get as much credits. All you have to do is select a driver. I personally suggest at least a 65% driver, but if you feel insecure about it, choose a better driver, but keep in mind that your earnings will be lower. Once you selected the driver, just start the race. Now watch a movie, do excersice, take out the trash, play with your pets.
Start the race, and then do whatever you want while you wait. If you have a wireless controller with charge-n-play cable, I would suggest taking out the batteries and using the cable, so the controller won't turn off due to inactivity.