Robyn -
I'll be off on vacation (with my bike) on that date - otherwise I'd be happy to do a long ride with you.
It's been a while since I rode the Seacoast Century, although I do ride in that area fairly often. It's mainly flat, and the century is set up pretty much as an out and back type of route. The interesting thing is that the start is not at one end or the other - but closer to the southern (western?) end of the ride. You can easily use your feelings as the ride progresses to determine your distance (as in, it's very easy to do a ride of any length - 100 miles, or 80, or?).
Of course I have no doubt that you can happily ride the full century.
--- Denise