Pretty much as has been said above, you dont need to put in the total miles of the race in a training run before hand, although a warm up race about 6 weeks before can help over a shorter distance (say a resonably tough 15-20 miler in this case). If you are going over Dartmoor I can recommend getting the hill training in. Going up and down, I find its the fast downhills that hurt more than the uphills!
As it is self navigating, then remember its better to check the map for an extra 15 seconds than run the wrong way for 15 minutes.
It is acceptable to walk. So don't kill yourself trying to run if your legs don't want to! Some people stick to a pattern like previously mentioned (25/5). Others take an a approach of run downhill and flat, any hill that you can't see over the top of then walk it! you will most likely develop your own method.
At the end of the day, get your key training in, any extra you can manage without detriment is great. Time on your feet is more important than miles. On the day just listen to your body, if it can go a bit faster then go faster. if it needs to go slower then go slower... just dont stop 
Oh... and once you have done the 30 prepare yourself for the fact you will most likely get home aching and look for a 50... and before you know it you will be running in the middle of nowhere as the sun comes up, munching flapjack and thinking "not bad, only a half marathon left to go and I'm finished!"
I will shut up now!