The ride that you speak of in the N. Georgia mountains will be doable if you get up early enough on the day of the ride. I would recommend driving to Helen, Georgia (<1.5 hours) and parking at one of the larger (free) parking lots on Edelweiss. From there it's a quick, flat warmup to the foot of Hogpen. You can suffer up that for an hour or so then ride over Jack's Gap. The next big challenge will be Brasstown Bald but if you're feeling puny I'd recommend skipping BB and finishing up with Unicoi Gap which will put you right back in Helen. I don't know the exact distance....my guess is ~50 miles. It will be hot. There won't be much wind and there aren't many stops for water or supplies. The good news is there will tons of cyclists up there in that time frame....as well as tourists, RVs, motorcycles... They tend to drive reasonably slow and give you plenty of room.

Good luck and have fun,

Michael