That's a tricky temp further complicated by the rain/wet. It will seem colder than it really is.
Some form of longsleeve top that is light and wicks moisture is key. I wouls suggest a singlet over or under this. A thin touque and gloves as well. Both the touque and glose can be taken off or put back on as your situation changes.
The legs are a tough call. That's borderline for tights for me. If you go with shorts, be sure to give your legs a good rub with vasoline. This will help keep them a little warmer.
At the start and for the first few miles, wear an old sweat-shirt and start the race with that on. Toss it as you warm up, and before you start to really sweat.
Fleck
Steve Fleck @stevefleck | Blog