There are many different answers available, the correct one depends on what you want out of the race. To finish? To be competitive? Time goal?
Merely running a certain distance doesn't necessarily have much value. It can be a huge psychological boost (I can actually run that far!) but it sounds like that doesn't apply here. The other obvious thing is that 30k on pavement is tough on the joints. Building up and having plenty of miles in your legs (on the same surface and terrain) is essential to prevent injury.
Beyond that, my training (for mountain biking and trail ultras these days) focuses on building your threshold before training long distances at tempo. Translation, get fast and push your aerobic limit before going far at speed. You'll train yourself to run the long distances faster, which is much more valuable.
This means you'll be doing a variety of intervals, after your base period, and then transition into long runs. Very effective, and you won't need to run overdistance or anything close to it. I did a 50 miler trail race this summer, and my longest training outing before hand was around 24 miles.