I ended up riding to Troutdale on the Marine Drive bike path, then took the Historic Columbia River Highway. It was a really nice ride. It wasn't too busy, and I was able to pull over often enough to let cars pass when needed (I am not a fast-downhiller). There is a short bit where you have to ride on 84 which is a little unnerving, but it was over quick. I made it as far as Cascade Locks, 65 miles from home!