I'm finding much more info about 2015 rides than 2016 rides, even those rides with annual events and January/February registration.
Have you done many large organized rides? Apparently many people enjoy them. Others get overwhelmed by the sheer number of people.
As mentioned, the STP is a 1 or 2 day ride, and hits a lot of Washington, and a little Oregon. But, maybe you could find something else in Oregon to follow it with. Or, perhaps start in Oregon, and take the train to Seattle, then ride back.