1 week before a half marathon and I have Achilles Tendonitis, what should I do?

I have been training for my first ever official run (sheffield half marathon) and Im pretty sure I now have Achilles Tendonitis. I know I've only done some online research but the pain in the back of my heel does seem to go once exercise has begun, and only really reappears the next day when I find I have a slight limp due to stiffness there. I know I should stay off it and rest while gradually lowering my exercise rate with that rest but with the half marathon approaching I was going to taper my training anyway...though obvious at the end I wouldnt rest but do the half marathon, but then I can rest my socks off right? :S