We did the Tierra Bella 100K on the tandem, 3500' of climbing with one real climb and a lot of rollers. This is not a tandem-friendly situation, but all things considered we did great with a 4:22 riding time. I had been really sweating the weather given the 3-4 stormy days leading up to the ride, but things cleared overnight and the weather was just fine, not getting over 65 deg or so at the end.
This ride, out of Gilroy, CA, has great rest stops with their specialty being home-made breads and baked red-skinned potatoes. The potatoes are so good it's a wonder we got out of the first rest stop. The ride also has more customary fare.