I havnt got that far myself, but i can answer a few of your questions judging from a few other people's posts around the forums.
Yes, when it says 24 hours, it really means 24 hours.
No, i dont think there is an option to save mid-race.
No, you cannot do a B-Spec driver switch like you could in GT4
If you are going to do an Endurance race, dont take it lightly and make sure you set aside enough time to do it.