I've done some long (100+ miles) day rides in Oklahoma in the summer. The temperatures were generally between 100 and 110 F, and it was humid. Based on my experiences, which generally left me more or less catatonic on the couch for several hours after finishing, I'd recommend not doing this tour. If you absolutely must go, ride at night and try to find some place out of the sun to sleep during the day. The one piece of good news is that it does get fairly cool at night in the desert.