I was stationed at Whidbey Island for almost 4 years. I used to know the area well. When I was there the Seattle suburbs ended at Everett. When I went back in 2004, they had extended halfway to Mt. Vernon. I imagine it is all the way to Mt Vernon, now.
it is pretty well populated all the way to Mt. Vernon. I am in a suburb in Shoreline which used to be semi-rural but has become more crowded. Sadly, the crowing is more apartment buildings and not more restaurants.