Plan to arrive wet with a change of clothes. If you wear waterproofs you'll just drench your clothes from the inside: Gore-tex and other so-called "waterproof breathable" materials can't dissipate 3 miles of perspiration, let alone 15 miles. Instead, find something that is windproof, stays warm when wet, and dries quickly so it's ready by the time you head out again.

+1 on the importance of fenders and mudflap. Road water is nasty.