Looking at the raw numbers you could pull off three weeks of 60 mile days , as it's about 1,200 miles between Astoria and San Diego. But this won't allow for any time off, so you can't wait out a day if the weather's bad, mechanical issues, or if you wanted to take a day off to either rest or see stuff, like explore San Francisco or hike in the redwoods. Do you have to do it in three weeks? If you have the flexibility I'd suggest budgeting for longer. Or if you have only three weeks to spare, maybe a shorter route, say Astoria/SF.