I live in the area, but I haven't ridden it - just sections here and there on assorted rides. It's mostly country roads or all country roads. Funny, I went to the OR state parks site and read the description - it does sound sort of like a bike path.
I think it's all just country roads, not very high traffic, I haven't ridden it, but have friends who have. They say it is a good, fun ride. Pretty easy. I'm up for riding it sometime if you need some company.