Another sea level dweller here. The highest paved road in Washington is 6,400 so when we decided to do a week long ride in Colorado a couple of years ago we were a bit worried. The ride started in Pagosa Springs at just over 7,000' and went over Wolf Creek Pass at just shy of 11,000'. The first few miles out of town every rise I felt the altitude, the power just wasn't there but by the time we hit the climb I'd adjusted my pace and the day went fine. The next day went even better even though we went over Slumgullion Pass at 11,500' and had over a 100 miles of riding.

What got to us almost more than the altitude was the heat. It had been unusually mild that spring and early summer in Seattle so most of our rides were done around 60F. All of a sudden we were riding in the 80's and 90's. My GF would get up an hour before me and be off with the first wave because she hates then heat more than I do. The riders from Colorado, Texas and other warm locales had an easier time of it.

Agree with what people have been saying, without 2 or more weeks to truly acclimatize you are better off just going for it as soon as you can and pay attention to pacing, hydration and eating.

I've climbed a few of the volcanoes around here (including Rainier at 14K and Adams at 12K) and the consensus seems to be try and get up them in two or three days at the most. Some people, strong ones, climb Rainier in a day but that is 9K of climbing with glacier gear.

Altitude does affect people differently at different times; I've had days when I felt good and others not so much. Also it can hit you very quickly so pay attention to how you feel. I've been hiking fast uphill a couple of times only to come to a grinding slow down at around 8,500K. It has gone better usually when keeping a more sensible pace.

You are lucky to live in state with so many great mountain roads to ride. We have a few great ones here but not near as many.