In reply to JamButty:
First up - enjoy it. Too many aim for a time of X or train so hard they hate it. You can work on improving your time if you do more later, but for the first you just want to get round and enjoy it. If that involves a load of walking that's fine.
Do you have a kit requirement? (waterproofs/compass/whistle/map/food/water) If it's trail I'm assuming so. Check what is required and get used to running (not all the time because bleugh, but some of the time) carrying that stuff. It does help to have some practice and know that your bag is fitting well etc before the day.
Get to know what you require to wear in different weather. I run reallly warm so I've run some coldish weather races with a vest and a lightweight waterproof that just meets standards, and know if it starts pissing down I'll be fine. Others I know run in so many layers with the slightest nip in the air that I would pass out from overheating! LEarning what you need means carrying less and that will help.
Get used to taking in fuel on the move. You don't need to run with gels and you may be fine running that distance without food, but it might help. At that distance a couple of gels and a quick snack at an aid station to give you a boost is probably helpful if you've got them. If you are not used to eating or taking gels though it could be bad for you on the day. Same goes for sports drinks if you normally use different ones or just water when training. Plenty of us have made the mistake of drinking or eating something new on the day and spending a few miles thinking we're going to have to dive behind a bush (which is very difficult on the Cross Bay half Marathon when I did this, as it's across Morcambe Bay and is completely flat! Thankfully realised what was causing it, stopped drinking, and recovered after a couple of miles of being very uncomfortable!)
Training plans always seem pretty high intensity to me. I'm quite lucky and tend to b e able to do more with less training than many, but more generally I'd say listen to your body. You should not be knackered all the time from training and being like that will lead to injury, hating what you're doing, and you being run down by time race day comes around. On the plus side, you don't have to have run a marathon in training to run one on the day. Work for slow improvements rather than sticking religiously to a plan that is exhausting you.
Feel free to walk-run-walk-etc on the day if you're knackered. we all have bad days, we maybe don't sleep well the night before, or we set off too fast in the excitement and find a few miles in we're knackered. Don't feel you have to push yourself to keep running as far as possible if a short walk might let you recover. And walk-run patterns move you faster than staggering run in to staggering walk and burned out by half way.
Back to the first point - enjoy it!
On the day you have the excitement, you have loads of equally excited people to natter to (and talking to people can eat up the miles very quickly).