If you have a friend, it's quite easy to do the 24h-races in about 24 hours. Me and a mate only did the 24h Nür on his savestate so far ....
You can do a two hours-drive in Nür until you start to get tired and lose concentration. One hour is quite easy to do since it's "just" some 5-6 laps, depending on which car you take....
We choose the Subaru Impreza Cusco-JGTC'04-car since it has good drivability, lots of grip and (with a softer suspension) tends to get around the track without bouncing around too much. You really want to choose a car with neutral handling, i might consider taking my Takata NSX for my savestate, though ...
Every two hours we "changed seats" - and it went quite well, despite some off-track-accidents caused by the AI-drivers.
There is one thing to keep in mind, though - make sure there is a place where you can rest for some 4-5 hours. Like Seiji Ara did in Le Mans, one can do a three to four-hours drive in the night if you did get up late the day you started the race. After that, be other driver can sleep some 4-5 hours as well and you really should rest a bit ... the last four hours are really exhausting.
And yeah, be sure to be able to use the right stick to brake/accelerate since your thumb won't be able to use the X-button without starting to hurt after 12 hours into the game. Especially if there's a long straight ...
Be sure to be well rested and have lots of coke, coffee or whatever you may need to stay awake around and you'll see how much fun it can be to have one 24h-race done.
If you have the chance, have a second PS2/cube/laptop/whatever around to relax from driving - it makes the 24h go by way faster. I watched Mystery Science Theatre 3000 (which made my mate doing some spins because of me laughing all the time, hee-hee) and played a bit of katamari damacy ... the main thing is - distract yourself in order to be able to do 12 hours of driving.
It takes some time to reach 100%, though ... but it's even more satisfying to have one down without using B-SPEC.