Seattle is pretty good for N/S routes, but honestly E/W leaves a lot to be desired. Some of that is terrain of course, but going between West Seattle and the aforementioned I-90 bridge is kind of a pain in the ass. Even the King County map conspicuously lacks E/W routes. Vashon Island is nice to ride, I do it frequently, and you can pair it up with Maury Island (linked by an isthmus) for a 50-60 mile ride. White Center is neither white nor central.