I have been on the ride in both 2006 and 2008, and suggestions that I can make ...

1. heat is variable and wasn't, to my experience, much of a factor in either '06 or '08. '08 was marked with a ton of rain in the days leading up to the ride, and the day itself was cool with a lot of general washout on roads. '06 was also, to my memory, somewhat damp and wet. what can be important, however, is that aside from the checkpoints, there are few, if any, shops or services available for resupply. So, at a guess, I would say that you should plan on having at least 3 liters of fluid on your bike (either large bottles or camelbak) and at least three hours worth of food (~750 calories)

2. Accurate navigation is fairly key, as there are more than a few unsigned forest trails that will penalize you heavily for taking a wrong turn. You'll be bombing down the side of one hill, take the wrong fork and descend for a couple of miles or so before realizing that you're off-course, and the only way to get back on course is to grind up a two mile, 8% grade.

I DNF'ed my ride in '06 largely because I made two wrong turns that just added an hour to my time and exhausted my supplies.

3. If you have any doubt about your abilities or training, take the 100k option. The 170k can be nothing but abject misery if you aren't on top of your conditioning for the season. It's not just about how many miles you get in on the summer, but what those miles consist of. Dirt? Climbing? Unsupported? Take Sandy Whittlesey's advice with "if you aren't doing 8 hour training rides with 10,000 ft of climbing, you probably aren't prepared" in considering the 170k option. The 100k option skips a few of the hills and gets in a lot of nice scenery as well as the covered bridge lunch stop. I know a fair number of guys, hardcore randonneurs who've done 1200k's and some fairly challenging fleches, who are perfectly happy with just riding the D2R2 as a 100k.