For my first half I'd only ever run 9 miles in training. Things started to hurt a bit about 11 miles in but once I got to 12 I knew I was going to finish. When you're upping the distance the last km or mile always seems hard. If you can do one 10 miler you should find the half fairly easy (as long as you don't go off too fast). From where you are now I'd be more worried about over training than not being prepared. Up your distance gently until the week before the half then enjoy it.