I think there's actually a fair number of people who ride the Cannonball as a warm-up to the Sea Gull.

Here's their website:

http://bikefred.com/CBC/cbc.html

The first time I did the Sea Gull the rain wasn't too bad because it was just rain. If you've got a good waterproof jacket, you'll be okay in those conditions. It was the nor'easter in 2006 that sucked - the combination of heavy rain and high winds were just no fun.