The John Wayne Pioneer Trail is well loved, and the western part is well traveled. The clearing site for all news about the JWPT is http://johnwaynepioneertrail.org/
26inchslicks rode the trail from east to west in 2012. His blog is excellent, especially the grueling report from day 2. http://26inchslicks.blogspot.com/2012/06/crossing-washington-state-on-john-wayne.html
Shawn Pedersen and his friend did a west-to-east crossing in the fall of 2015. His planning blog is here. It’s got great info for those planning the bike the trail. Below is some beautiful drone photography of their trip:
And here’s a great 15 minute video of a mountain bike ride from west to east along the John Wayne Pioneer Trail. This gives you a good sense of the trail conditions (smooth to rocky to overgrown!) and the landscape.