Your clothing suggestions are very much what I thought. I do get very warm when I run, but rain at that temperature could mean slight hypothermia for me. I was actually thinking of a very light wool LS base layer, where I could push the sleeves up. I do have a LS PI running jersey with a 1/4 zip, but it might be heavier than I want. I was thinking the wool would keep me warm in the rain.
I hate wearing hats, but I have a long visor running cap.
I haven't run in a week, but somehow I am looking forward to this. The last time I did this race, my time was 34:00 exactly and I think I am way ahead of this, but who knows. MY goal is 30:00.