Sounds like a great adventure Al. What are some of the previous rides you've done?
Your outfitter certainly has put you on a lot of western interstates. There's local roads too. But they're not graded like the interstates, so are a lot more work, going up and over each hill and down into all the valleys. The interstate grades everything smooth, using the cut material from the hills to fill in the small valleys. But I think the extra work is worth it: no pollution, noise, shoulder riding with all that debris, and 18-wheelers doing 80mph passing within 3'.
I did much of your route from Cottonwood, AZ going west in October 2004 by myself. And yes, I arrived in Blythe, California on I-10. But I cut off, going south along the Colorado River, across the desert to Brawley in the Imperial Valley. And from there, over the Coastal Range to dip my wheel in San Diego...mostly two-lane roads.
For some of the ride, I used maps published by
Adventure Cycling...my route was the black line on the west part of this map. They use interstates only as a last resort. They send out research vans to find low-traffic local roads and scenic countryside to use for their routing. I recommend them.