I plan to take the Kirkland waterfront or 6th/108th from downtown Kirkland south, then onto 112th to downtown Bellevue, then the bike lanes on 118th and trail combo down the Lake WA Blvd to Renton Boeing. Then Logan bike lanes to Cedar River bridge.
My destination is in Kent right smack off Interurban trail.
Question is, and I've been wondering about this for years for my training rides too, is how best to get through downtown Renton to reach Interurban? Rainier Ave and Grady Way is how I usually do this, but they are super sketchy. I don't want to ride on them every day.
7th Street is the typical way that people get across Rainier. I take Park south from near Coulon Park. Take 7th all the way to Oakesdale and crossover like you are entering the sewage plant. There is a small trail on the left. You can follow this path all of the way as it snakes around near the Kiddie Casino (Family Fun Center). Or most people take Longacres Way to the right which goes under the freeway and down to the Tukwila train stop. Go carefully through the parking lot under the train tracks and the interurban is right there by the Hotel.
You want to stay on the East part of Renton till you hit 7th.
Right when you cross the train tracks and hit the stop sign turn left onto Houser Way. Right on 4th St then Left on Williams is a nice quiet street though the middle of Renton till you get to 7th and then follow the above advice from ChinookPass.