A lot of desert riding today - not a ton, miles-wise, but when you're out in the heat, it adds up.  If I've learned one thing during this trip, it's to not push it when it's hot.  Drink plenty of liquid, make frequent stops, and don't be afraid to call it a day whenever you feel like it.  There's no way I could have done an Iron Butt in the really hot temps that I've experienced in the later portion of this trip.