I'm at almost 7 hours of the 24 at Le Mans with an Audi R10, boosted to 788 hp, almost 3 minutes in the lead. Even though my Bobs (lv 33s) have to stay out at least 2 stints after their strength is gone completely, just to give the others a chance to rest. Anyway it's more realistic to not change drivers every time the car comes in.
We just have to squeeze the most out of the two main factors going for us in 24H races.
1) The new tire wear that tricks AI drivers into staying out for just another lap, even at Le Mans that can be an issue, to say nothing of the Nurburgring.
2) AI drivers wear themselves out entirely as the hours pass but the game doesn't tell them to not pit, or take it easy when they get caught in a dogfight with another car.
The R8s and the Pescarolo C60 Judd '05 are the main threats. Failing those, maybe the BMW V12 LMR, or a Bentley could cause trouble.