Hmm, it is a tough one,
In GT4 I A-spec'd all of Nurburgring and La Sarthe, playing for a bout 6 hours a night for most of the week on each, just pausing the game and praying for no power cuts between each session, in that respect I am not 'hardcore' enough to claim I think saves are a bad idea and only those that do it in a real 24hr's should get the top prize, but, I do agree with the point above..
If you take the "endurance" out of endurance racing, then you are just racing a normal championship race.. 5-10 laps.
I doubt PD would be able to release a game that REQUIRED people to play none stop for 24hrs straight, so the pause button is a given... therefore the HARDEST an enduro can ever be can involve pausing every now and again for sleep etc. even if it takes two weeks... it is do-able.
Being able to save during the race really takes the edge off the race, if you fudge up, you can always go back to the last save. And I don't think you should be able to do that... so maybe the console should save your progress at 6 hour intervals, but not allow the save to be re-loaded more than once.
Another idea could be that the console monitors how long you are playing for between pauses/saves, and the more chunks it is broken in to, the less of a reward you get.
For example:
12 × 2 hr stint = disappointing prize car 100,000cr
4 × 6 hr stint = average prize car - 500,000cr
3 × 8 hr stint = good prize car - 1,000,000cr
2 × 12 hr stint = excellent prize car - 2,000,000cr
1 × 24 hr stint = excellent prize car - 4,000,000cr
..and maybe you have to nominate the strategy before hand.
I'm not an elitist. I'll play GT5 like I have every other installment, SD-TV, standard controller, automatic, few/no driver aids, bumpercam.. and I will enjoy it, but sorry.. anyone that thinks they have conquered an Endurance races by breaking it in to managable chunks (edit

as much as someone who as done it in one sitting, is majorly deluded...
... it's an endurance race, it should be difficult, physically uncomforable and tiring by the time you get to the end of it.. that is the challenge! Otherwise it is NOT about ENDURANCE!
.. and B-spec should be removed
Multiplayer would be a great option, but only when your team are the only real people in the race, I can see that being pretty easy to implement by PD (even if you and your mates had to co-ordinate via phone etc.).. where it might fall down is against other 'live teams'. With only your team racing the game can pause itself (the other cars) to catch-up/transfer data etc., if necessary - i.e., if the server detects a connection drop it can pause/wait 5 mins before dropping the race to give you time for you to phone your mates and get them back into the race... that couldnt be done with multiple online real teams.