As others have stated, you shouldn't do any real training. This is your taper period, and you'll do much better with fresh legs at the 1200k start.
Before a 1200k I always do a few things:
1. Bank up on sleep. Get an hour or two more sleep per night whenever possible, and force yourself to sleep in a few days before the event. This will pay off during the event or in helping cope with the travel and stress of prep for the ride.
2. Take it easy on the coffee. I normally drink coffee all day at work, but this means that when I need caffeine at 2am during an event it has barely any effect on me. Getting some of that tolerance out of your system will help it work when it counts.
3. Stretch often. Should always stretch, but I frequently get lazy and don't, I make sure to start again for a bit before every long event.
4. Eat ALOT for the week or so before. People carbo load for 2 hour events. Imagine how much you'll be buying through in your 70-90 hour effort.
5. Get all your stuff together now. Realizing you forgot something two days before the event sucks.
Good luck!