I've finished it twice and bailed on a third (it was raining pretty bad from the start in Seattle, and we didn't feel like being wet and miserable all day).
The route is really easy. There are only a couple of hills, and they are not really that bad. Based on how much you say you ride, you shouldn't have any trouble. If you are worried, you might try some of the spring tours to test yourself. I know there are a few around there put on by groups like the
Portland Wheelmen.
My main suggestion would be to ride with a friend or two, if possible. I think it can be really boring without someone to chat with.
Average speed for most people seems to be something like 12mph. Pretty slow. I think my uncle and I would average 16-18 not counting time spent at rest stops. Tandems and speedy guys seemed to be doing 20+.