LateLate - The longest ride I put in before my first metric century was 51 miles (about 13 miles shy of the metric distance). The course was extremely hilly, but I really had no problems save a bit of fatigue after a series of "one hill after another" about 2 hours into the ride. Before the ride, I had been logging 200+ mile weeks, though, and I really worked raising my lactic acid threshold. Sounds like you've been training smart. You'll be fine---just take your time, pace yourself, listen to your body, etc.
The good news for you is that the Seagull is a flat course. The winds could be an issue, or so I've heard, so be ready for that.
Maybe I'll see you there. I'm riding the metric -- hope to bring my pink Luna Eclipse but if there's rain predicted, I'll be riding one of the "rain" bikes (Specialized Dolce - probably the blue one w/the pink tires). ;-) Looking forward to that pie & ice cream rest stop refuel!!!