Dan, I did my first Half Marathon in Sept 2011 (1:57) and that was my first ever race and then did Manchester 2012 (4:06) as my first marathon. You will be fine. You will also find that you get your Half PB down a fair bit from marathon training too. I knocked 13 mins off mine in 6 months just from the extra endurance. Maybe look into a training plan?
Colin, who are the haters? I think people have the right to be annoyed. It wasn't just the chip timing which has frustrated people, the fact they have the most diabolical chip system ever known to marathonkind, you were tripping over a load of them on the route, was dreadful and then water in cups, again, for a city marathon, is just not cricket. I expect that in a local club race which I paid £4 for, not a £40/£50 race. I think being a race director is a very tough job but this isn't rocket science.