If the race is to be on the track, definitely get some *substantial* training runs on a track. Eight lefthand turns/mile really starts to add up.
Different things work for different people, but I think a lot of people in these super-long races are best served by mixing, right from the start, running (easy running, but running and not shuffling) with occasional walking breaks of a minute or two. Take a walking break before you "have" to, rather than getting to the can't-run-another-step point.
I've been told that protein is important in these very long endurance tests. This confirms one runner's account of his 24, wherein he said that chicken broth was delicious but a banana was "repugnant."
As far as training is concerned, for my ultras I always felt that I got better conditioning from going out at a decent pace for an hour or somewhat less--but typically two or three times a day--rather than slogging through individual long efforts. I found that the long training sessions tended to take a lot out of me and had a hangover effect on the next day or two, whereas I was able to recover overnight from the two- or three-a-day shorter sessions. But my longest races were only in the six-hour range--perhaps my attitude would be different with training for a 24.
Anyway, good luck. It should be one of the most memorable events of your life.