Hwy 18 is unrideable
Last fall I attempted to ride from Issaquah-Hobart road up over the section of Highway 18 to link up with the Tiger Mountain trailhead.
Hwy 18 is essentially a freeway with fairly heavy truck traffic. The shoulder goes absolutely to zero. It's 6 inches from right elbow to steel guardrail, 6 inches from left elbow to semi-truck. As soon as I hit that section I bailed out. You should be able to see this on Street View about 1 mile north of Issaquah-Hobart.
It's much safer to ride on Highway 90 than Highway 18 in my opinion.
The other south-to-north option is to head east on 208th street, and you can link up to Rattlesnake Road on gravel/logging roads. Again, requires something with fatter tires.