We're neighbors...
I'm in Pendleton... also a "clyde." I recently rode my first metric century (62 miles, 65 after a wrong turn). I was the last to finish after 4:49, averaged 13.5 mph. About 1.5 mph slower than I expected. It was 96 degrees and I did finish. The SAG vehicles were full of riders cramping and konking out of the ride. The organizers of any of these rides are not surprised to have a few big turtles bringing up the rear. They will hold the rest stops for your safety also.
We have another ride you might be interested in trying.
http://www.ride4raptors.org/ It will be cooler for me too!