This is a route that will start you from downtown Portland (from mile 28) towards Astoria. This specific leg ends at Big Eddy campground about 10 miles NW of Vernonia. From Mile 28 on this map to the end is the best route I've managed to find from downtown Portland to Astoria. It goes by way of Hillsboro to Banks to Vernonia and a bit beyond. The route is all bike friendly and relatively scenic in the developed areas (and great in the rural areas). Pavement is great, bike lanes are good in areas where traffic is meaningful. Services are limited after Vernonia so you should fuel up there (there is water/toilets at Big Eddy Campground and the Jewell Elk Reserve). Vernonia has a number of good restaurants and a decent market for food for the next day. Big Eddy Campground (about 10 miles NW of Vernonia) has bike camps for $7 per tent plus a $5 registration fee (for the entire group) and $5 per shower (which are very hot and untimed). This is a good bike campground.