Congratulations on making that decision, Julia!
March 2009? Loadsa time!!!
You've got a good base to build up on, as others have said, some people run half maras without even having completed anywhere near 8 miles in training!
I'd actually recommend trying to get further than 10 miles in training before running the race. The closest you can get to the actual distance, the more comfortable you'll feel on the day - say, 11 or 12 miles (I actually got up to 14 before my first half, with the intention of making the actual distance seem 'easy' in comparison). You have plenty of time to still increase your mileage by 10 % per week and get up to this sort of distance. Focus on distance and increasing your endurance and 'time-spent-on-feet' first, before worrying about speed (although given how far you're already running, a little variation to the types of run you do probably wouldn't hurt).
As Mick suggests, it might be worth entering the odd shorter, local race to gauge your performance in the run-up to the big event, and also to get used to races, what happens, how it all works and how it all feels.
Anyways, the best of luck with it - I'm sure you'll be just fine.
LP xx