Is there a time limit? could you do it over several weeks? 😀
I did the Ronda 101k 24hr race last year. The weather was awful.
The best advice I can give is just keep moving your feet in the right direction and switch your brain off.
A part from that, start slow and ease off, don't panic, eat little and often, run your own race and ignore what other people are doing, use lube, be aware what is happening to your body, take lots of supplies and an ipod/shuffle, a good head torch and lots of batteries, maybe a small emergency kit.
Also make your self aware of any dangerous animal which might be around if it's off road.
This site has lots of tips.
http://www.ultrunr.com/
It is good fun and you get to see a lot. You will have a great time (as long as you don't puke 😀 )
Break a leg.