Just curious why Walla Walla? From WW you could head to Seattle then from there follow the Seattle to Portland route as far as across the Columbia at Longview then west to the ocean. Meandering back east to Crater Lake would also be worthwhile although doing all of the above will add I'm thinking at least a week to your tour. From Crater Lake to San Francisco is an almost direct shot down 97 and 5.
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Sir Mark, Knight of Sufferlandria