NWC -
Glad to hear you are going to ride X-USA, but you don't give us much to go on.
I doubt you are 19, but are you 45? 65? 85? It makes a difference.
Also, how much riding/touring experience do you have? The two are different, btw.
(But if you have a sag wagon, you probably won't carry more than a granola bar.)
When are you thinking bout doing this? Starting in May or August could mean different routes.
Although it's not a designated ACA route, one route with the most gradual climbing
starts in Westport, WA via Raymond, Chehalis, Packwood and White Pass.
Don't be fooled by the Columbia River - lots of climbing and fairly busy roads.
The TransAm also has a chunk of climbing getting over the Cascades.
If you don't overdo it the first week, you will settle in fine.
Best - J