If your doing the c2c bit, I can tell you that on sunday we ran into three guys that were doing their final 1/2 mile to the summit (still five miles to go back) they appeared to be very fit and they were in their late 20's early 30's. they had already lost 6 people from the group that stopped at LV. They weren't smiling at all, and appeared to be pretty worn out. I believe they said they left the museum at 5, so they spent some time in the sun and it looked like it was really getting to them.
I agree with Kathy, earlier is way better now, and maybe 7 liters is a good idea. you can always stash some up on skyline when you start to realize you might be safe ( you remember where right ?

)
good luck, and maybe I'll see you on the trail. -steve
this past weekend on skyline I went through 3 liters of water and 1 1/2 liters of cytomax and I wasn't in the sun but a couple hours.