Topspin: I think you have made a smart decision. My wife and I did the Haleakala Volcano Bike Ride in Sept. 2005. It was a shore excursion through the cruiseline (RCI). It wasn't a sunrise tour, but it did last over 7 hours. A van picked us up at the tender pier, picked up the bike trailer and up to the summit. We suited up in 38 degrees and floating rain; and began the 38 mile downhill excursion, ending up in 85 degrees of sun.
Here's the bad part: we had a guide who didn't understand the laws of physics. She put the lightest riders in the front, with the heaviest at the rear. And yes, I was one near the rear of the pack, constantly using my brakes. BTW they use heavy beach cruiser style one-speed bikes with drum brakes that are NOT adjusted properly. My wife on the other hand was 2nd in line, and rarely had to brake.
I ended up with really sore (but stronger) hands, a few blisters and several close calls with other bikes whose brakes worked better than mine. Do yourself a favor and don't bike down, drive down.
Have fun, Steve