hmmm, I might be tempted to do this route, if I just wanted to test my gear (camp in Larrabee State Park). Way short of your desired distance though.
http://tinyurl.com/3tha7hr
I will admit that I have never ridden these roads, but chuckanut is classic, and the others look alright.
This route for a few more miles?
http://tinyurl.com/3l2v3uk
Maybe someone with more knowledge of the area will chime in. I am interested in the answers as well.