I've ridden all the roads you're planning. The trip down the coast will be outstanding, I've done that three times. You'll probably encounter damp, cool mornings and overall great weather that time of year.
From the time you get about an hour outside of San Diego, until you get about 2 hours north of Phoenix, you will absolutely cook. Unless it is very early summer expect temperatures well over 100. If you're not accustomed to the heat, be very careful. The back road up from Tucson, as mentioned, is a nice alternative to I10, in fact it ends about a half mile from my house.
Once you get up north, the weather will cool significantly. If you're here in mid July or later watch out for afternoon thunderstorms. They can spring up very quickly and get quite violent.