Steve, I did this ride about a month ago. I choose a route that was in part, the Willamette Valley Bike Trail you referenced, plus parts of the Pioneer Century, bits of the Enola Hills ride, recommendations from friends, etc. I can send you the route if you like. The total distance was 142 miles, with about 4000 ft climbing. I took the bus (Trimet) down to Wilsonville - I saw no reason to hack my way through Portland to get to the good bits. This is a pretty easy ride, since at this time of year, you get a tail wind in many places. On the other hand, it could be hot, and the last 50 miles is exposed.

There isn't a lot that I would change, although there are the inevitable short sections of busy road.