Flagstaff to PHX would be a awesome ride...but not down the interstate! Or to put it another way, why would you want to bypass some of the finest cycling roads in AZ to have traffic with an average speed around 80 mph blowing by you all day?
Flag, Sedona, Jerome, Prescott, Yarnell, Wickenburg and then a few options into town would be a dream ride! Plenty of Hotels and places to eat along the way.
Too much milage for me, but I've ridden enough chunks of it to know it would be fun for someone looking for a good ride in AZ.
And for you MTBers, you can do it on dirt as well! I don't know how to connect the Mormon lake area with Flag (Off road), but Mormon Lake to Cave Creek can be done on nearly all dirt.