Oh also forgot to mention one thing. I beat the first race, which is the 20-mile Willow Springs race (8 laps)
The tires went bad very fast. I was using sports soft and one of the tires was almost all black by the 4th lap. Same was with the fuel, I only had about half on the 4th lap. It felt like they were trying to give you the feel of long races but make it a lot shorter.
The leading car was a FXX, which I gott admit wasn't easy to catch and it sort of kept up with me after I overtook it. So it felt challenging. Then again, I was using the Ford GT, which is a real pain to drive with 650 PP (that was the limit).
One more thing: I drove the R8 LMS Ultra and it was very, very hard to drive. It spins way too easily, even with racing soft tires and when you break hard it just loses control, unlike in GT5. I had to drive it very smoothly to not spin out.