There are several bike trails from San Diego CA to Yuma AZ and these are really challenging, especially considering the mountain crossing. You go from sea level to an altitude of over 4000 meters. It's a wicked ride with lots of hills and heat. Often there are strong crosswinds and you get quite banged up on the bike. The route is a little challenging for the first 22 kilometers, but may I suggest you head to Hotel Del Coronado to help you along the way. The wind along the Pacific Ocean is lush and always blowing.
Now departing from the Del Hotel you travel down Hwy 75 south for about 9 miles and a wonderful wind-driven ride in the back. This will be a while of beauty, the warming will be nice given the future ahead. Once you hit Palm Ave. you are ready for your trip east across the bottom of the country. Take Palm Ave down Beyer Blvd north to Orange Avenue East, which turns into Olympic Parkway. Turn left onto Hunter Parkway 1 mile to East Otay Lakes Rd, then 7 miles to Hwy 94, which you will endure.
Then you will drive Hwy 94 to I-8, about 50 miles of arduous driving, dry, hot and long, and before you arrive you will drive Old Hwy 80 and about 10 miles, which then make a little connection on I-8 for about 10 miles over the 4,000-foot crossing, it's really one climb. Next you will get off at Hwy 98 and drive up to Calexic for about 38 miles, but it is generally all downhill and refreshing, good considering the dusty desert and winds. But now you are in apartments and can make a good clip through the desert.
After Calexic, drive another easy but rather boring 20 miles to Evan Hewes Hwy, which is like a front road for a while, then you will come to 186 Hwy and head north to Araz Junction and follow Araz Rd. to Yuma Crossing State Park, from where you will be driving to Yuma, AZ at Business 8 or Fifth Street. You've walked 177 miles, not bad for a 12-hour ride, may I suggest a city cruise to Barnes and Noble, as it's a great pickup point with an air-conditioned cafe, and in the meantime I'll be gearing up for my second day of driving to Phoenix on my ride America.