In the "connecting routes" section of the forums, I gave some pretty specific directions for getting to Seattle from the NT route east of the town of Sedro Woolley (which is on Highway 20, as is Anacortes, but about 25 miles east) but that is somewhat out of the way. Basically, there is a light rail train to downtown Seattle from the airport. Heading north from downtown brings you to the Burke Gilman Trail. Head east on the Burke Gilman to the town of Woodinville, then go norht on 156th. Stay off of Highway 9 (Google maps bicycling function will put you on it--they call it Snohomish Woodinville Road). In the town of Snohomish, find the Centennial Trail (Off Maple St), and ride it to Arlington. In Arlington, you can get on Highway 9 and continue north. You will need to bear left at the roundabout to get to the town of Mt. Vernon. Go north out of Mt Vernon to Burlington, where you will come to Highway 20. Go west on Highway 20 to the town of Anacortes, which is actually on Hwy 20 Spur. The ferry terminal for the San Juan islands is west of downtown. When returning, Where the spur and the main hwy 20 join, go south to the Keystone-Port Townsend ferry. Take the ferry across the Sound, and you are on the Olympic Peninsula.
I live in Snohomish about 4 miles from the trail.