Michael Wolfe posted a route map awhile back detailing how to get up to the Longview area paralleling I-5 -- haven't ridden it but he's a reliable reporter, methinks:
http://escapevelocipede.blogspot.com...o-seattle.html
Clark County has a new stealth bike map (they don't tell anyone it exists) - here's a link to the pdf but there are paper copies available somewhere, maybe...:
http://www.clark.wa.gov/public-works...h/Bike2007.pdf
Finally, north of Longview i do know a little something -- hwy 411 on the west bank of the Cowlitz up to Vader is pretty good for bikes, and from here if you've got the time and inclination going west toward Ryderwood then north toward Bouisfort & Curtis is an outstanding ride, very quiet -- tho it does involve a goodly climb and adds maybe 10 miles. From Curtis you can head in to Chehalis.
ok, enjoy!