I had trouble with this series at first too, trying with the CLK-GTR. I wouldnt worry about qualifying, no need to be in front right off the bat, they are long races.
Im currently running it with a stock Minolta Toyata, I'm on race 7, won the first 6. I've got very hard's on the back and hard's up front, and set the TCS at 2 for tire wear and ASM at 0/0. It has been pretty easy so far, usually winning with a 30 sec or so lead. I just go all out and try to build up a big lead until the rears start to turn orange-usually about 14 laps. By this time, the other cars have usually pitted already, giving me a huge lead and time to pit myself without them catching up. Some I finished without pitting but for the long ones I stuck to pitting every 13-14 laps or so. Also, make sure you exit to garage and save between every race just in case!
But I wouldnt try with the CLK-GTR, it just doesnt seem to have the turning radius of the other cars like the Minolta. It can keep up on straights but the car is just sooo long, I think that affects its cornering.
The El Capitan endurance is pretty easy too, did it in a stock Amuse S2000 GT1 only took around 2 hours, definitely worth it for the Toyota.
Oh yea and I kept restarting it to get an easier field of cars, do this, it helps...the set I stuck with was Toyota GT-One, Chaparral 2J and 2D, a Nissan R390, and the BMW McLaren F1GTR. I thought the BMW would be the one to worry about but the standings are : Me 60pts, 2J 29pts, GT-One 28pts, Nissan 20pts, BMW 13pts, 2D 6pts. 👍
Good luck!