1st get a BIG  garbage bag to keep you dry before the start. Just poke a  hole in it for your face and stay warm & dry underneath...you will be so jealous of the other garbage bag wearers if you don't have one!!

I have done two 1/2s in on & off rain and a 10k in a MAJOR down pour!! Layer (disposable/good will clothes) to keep warm at the start and make sure your clothing fabric is wicking from head to toe. I also suggest a running cap to keep the rain off your face.

Good luck & Happy running!