I don't have a specific route planned. I'm tentatively planning on ending up in Portland, OR, so I should stay north for the most part. I'm also planning on bringing along a good road atlas which should help in providing direction as I move along.

Sometimes a lack of planning is refreshing and leads to instances of serendipity that you could never achieve by careful planning. However, I suspect that in this case your noticeable lack of planning will quickly lead to a total breakdown of your trip, as you now envision it. Either that, or you will need to learn how to succeed in this undertaking through a highly compressed crash course that could be very stressful as you struggle to master the unexpected complexities of this trip you now think of as 'simple'.

Whether you walk across the country as you now intend, or take the advice already offered above, of switching to a long-distance trail like the Appalachian Trail, I'd urge you to read as much as you can about the experiences of people who took similar walks, paying special attention to the nuts-and-bolts practicalities of making it all work out. Otherwise, one week after you start on this months-long trip you may find yourself stymied and not able to continue.

I wish you luck, but remember that "luck favors the well-prepared".