There are only a couple of bike routes doable from San Diego CA to Yuma AZ and they are indeed challenging, especially consider the mountain pass. You go from Sea Level to just over 4,000 feet in elevation. It is a wicked ride with lots of hills and heat. Often there are severe crosswinds and you get rather sandblasted on a bicycle. The route is a little tricky for the first 22 miles, but may I suggest you start out at the Hotel Del Coronado to help work you into the route. The wind along the Pacific is brisk and always blowing.
Now then starting from the Hotel Del you travel down Hwy 75 south about 9-miles and a beautiful ride with the wind at your back. This will be the last of the beauty for a while, the warm-up will be nice considering the future that lies ahead. Once you hit Palm Ave. you are ready for your trek Eastward across the bottom of the state. Take Palm Ave to Beyer Blvd North to Orange Avenue East, which turns into Olympic Parkway. Turn left onto Hunter Parkway 1 mile to East Otay Lakes Rd and then 7 miles to Hwy 94, which you will endure.
You will then take Hwy 94 to I-8 about 50 miles of hard riding, dry, hot and long and just before you get there you will take Old Hwy 80 about 10 more miles, which then connects to I-8 for a bit for about 10 miles over the 4,000 foot pass, that is one heck of a climb indeed. Next you will get off on Hwy 98 and take that to Calexico about 38 miles, but it is most all down hill and refreshing, well considering the dusty desert and winds. But you are now on the flats and you can make a good clip across the desert.
After Calexico you ride another easy, but rather boring 20 more miles to Evan Hewes Hwy, which is like a frontage road for awhile and then you will come to the 186 Hwy and go North to Araz Junction and follow Araz Rd. to Yuma Crossing State Park, where you will ride into Yuma, AZ on Business 8 or Fifth Street. You made it 177 miles, not bad for 12-hour ride now may I suggest cruising through the town to Barnes and Noble, as it is a great pick-up point with an air-conditioned coffee shop, Meanwhile I will prepare myself for my second day of riding into Phoenix on my Ride Across America.