Matt - numbness is pretty common in endurance cycling. Typically, the nerve supply to the palmar side of the hand is as follows: median nerve (carpal tunnel nerve) thumb, index, long and half of the ring finger. Or, the ulnar nerve which feeds the other half of the ring finger in most of us and the pinkie. Since it sounds like this is a first time for you, and only in the longest of rides, I would consider just watching it or, at most, having enough padding for your palm (gloves and bar tape) changing the position of your hands frequently, shaking out your hands occasionally, that sort of thing. If the problem recurs/persists talk it over with the folks at your LBS before seeking medical assistance.
John