If you read some of the journals about the American Discovery Trail that i suggested (the ADT runs from Delaware to just north of San Francisco), you'll find that even on a recognized trail there were numerous times the hikers had to put up at a motel because there was no place to camp. Often they had to ask permission to camp on private land, which was not always given. Some had serious confrontations with local law enforcement. "Lion King," who hiked the ADT 4-5 years ago, was mugged in East St. Louis. "Boston and Cubby," who hiked it in 2012, had an easier time, except for the aforementioned problems with their water caches. With only a few people hiking the trail each year, it's not that well known to the local inhabitants.

Out in the west, in many areas the only through roads are interstate highways, and it's illegal to walk along those (believe me, you don't want to anyway, unless you enjoy trucks blasting by at 70-75 mph every minute or two!). The ADT is mostly road-walking, but at least it avoids major highways. You also need detailed maps so you can distinguish private from public land; otherwise you may confront irate armed landowners. And settlements of any kind are often many, many miles apart.

There would be several options to hike on to Portland. One is to hike the ADT as far as the crest of the Sierra Nevada and then follow the Pacific Crest Trail north to Cascade Locks (just east of Portland).

Note that you need to time things very carefully to get across the Rockies and Sierra Nevada during the few snow-free months of the year. Timing will be even more critical for the alternate I described. You're basically talking June (when most years there are still many feet of snow left, requiring snow navigation skills and gear) to mid-October for the Rockies, Sierra and Cascades. If you wait until late spring to start from the east, you will probably have to lay over somewhere during the winter months and then wait for the snow to melt in the mountains.