I did 3 out of all 4 of the A-Spec/B-Spec 24 Hrs races.
For B-Spec, it's basically bringing out an X2010 and winning with an impossible distance (FOR ME).
I've tried once with the Pescarolo, but you'll have to click the 'Pace Up' button for most of the race in order to win, so I do not recommend it.
For A-Spec, you can either choose the above method, or race with a car that has performance similar to the AIs (mostly LMPs). If you decide to race with a X2010, you might get bored after a few hours. But if you race with a LMP, you'll need skills to win (unless you cut corners, just like me

). So it's about if you only want to complete the game, or if you want to have some real competition.
As for the Nurburgring 24 Hours, err... I'm still trying to make a tune for my Schulze N24
