Last summer I rode from western N. Dakota to Nashville, TN. Afternoon temps were 100+ each day so I kept the days to an average of about 550 miles per day. Three things helped greatly in that heat -- a Camelbak bladder in my tankbag that I could reach while riding, the STen's wonderful cruise control, and my trusty Corbin saddle. I would on average consume about 100 ounces of liquid each day, and surprisingly enough, after a shower and a few dozen ounces of beer (reward time!) I would feel reasonably fresh. I'm 53 years old, but I do a lot of bicycling in the summer and I think that helps condition your body to the heat.
On my final day on the way back from TN, I rode from north central Missouri all the way home to western N. Dakota in one sitting of 950 miles, but the weather was much cooler once I reached S. Dakota -- in fact, it was downright chilly. In the interest of full disclosure, I can't really say I was 'comfortable' for the last few hundred miles of that day as you do tend to squirm a bit after that many miles in a day.
I sometimes will pound a Red Bull while I'm riding long distance as it does seem to help keep me on my toes for long stretches. Funny enough, I never, ever, drink Red Bull at any other time except when I'm riding. I don't know if it's placebo effect or not, but it really does seem to help.
