Not all of us, despite our desires to be racers, have the patience to run 4 straight hours of, effectively, time trialing. GT5 satisfies the itch, but can't do everything right. The lack of a second driver for the 24 hour Enduros is a key point.
As for me, what I ended up doing was putting a Supercharger and Sports: Mediums on the Roadster prize car I got earlier in A-Spec and went to town. Easily faster than the other cars and the tires lasted me a good hour before I'd have to pit and change them. At that point, it came down to when I started getting a little bored, I'd hit pause and go do something else. Browse the forums or watch youtube or something.
I've heard people marvel at the "half'n'half" work method and how it improves efficiency, despite its apparent inefficiency. Basically, it's a half hour of work, followed by a half hour of whatever you want. If you're getting bored with the enduros, then every once in a while, pause and do something else off the Playstation. Then go back and do it some more.