First, never ever change anything in the weeks leading up to such an event.
Second, if you're tag-teaming these, then whatever, but if you're solo'ing, then the only way to prepare is to actually put in long saddle days. Finding the right FIT for you and your bike and properly fueling (food, water, electrolytes) your body is more important than anything else.
My uncle rides a lot of these enduro races (Trans-rockies, La Ruta...) and began his training a year before his first race!! That's from a non-sports type person (see:lazy) to La Ruta in a year. Things can be done in a short period, but it will HAVE to be grueling and well done. Rest time is important when training uber-hard, recovery is the way to success in enduro racing...
Well, I am certainly no pro, but I have done a few solo 24 hour races with & without success.
First, I would suggest riding often and at all hours of the day & night as you will be required to do so when racing. You'll also need to find out what does and doesn't work for your stomach in a variety of conditions (heat, cold, rain, fatigue, time of day, etc.).
I would also recommend chamois butter and really good shorts (I have had much better luck racing with lycra than baggies as there is less movement & chafing over time).
You may also want to look into and try some sort of electrolyte replacement. I have been using the LUNA Sport products and love them thus far, but I have also had excellent luck with Elete as Heed and other flavored kinds don't sit well in my stomach over a long period of time.
Definitely have a good wrench & someone to support your food/hydration needs. There will definitely be times when you don't want to think about eating, but you will need to and you'll need to have someone there that can help with food prep and reminding you to eat.
Bring spare parts: tubes, chain, pedals, shoes, drivetrain, tires, etc. Things break and get worn out over the course of a 24 hour event.
Invest in a good lighting system if you don't already have one. Spare batteries and a quick charger are also quite handy.
A celebratory beer (or three) for the end of the race is also highly recommended...trust me, they never taste better
If you have any other specific questions, let me know.