Lizzie,
my first race was a half too. I never realised how great it is running with loads of other people. I trained on a 12 week programme, 5 days a week where the longest Sunday run started at about 3-4 miles and increased by about a mile or so each week up to week 10, peaking at 12 miles, then it dropped back a bit to allow for recovery the two weeks before the race. Up to the point where I started the programme I was just concentrating for a couple of months on getting into the habit of running regularly and slowly building up so that I was ready for the programme, so you have plenty of time. Get the right shoes for the job and don't make the mistake I did of wearing the wrong ones when the mileage started to climb - you don't want shin splints. I'd agree with what Richard said about doing a couple of smaller events along the way if you can. Looking back, I wish I'd done that just for the experience. Good luck with the training, there are plenty of programmes around on the internet if you do a google search - the feeling of crossing the line is fantastic
