Jon, you will be fine.
Remember, don't worry about keeping up. The group should wait for you at every turn! If they don't, firmly remind them that they should
Slow in, fast out!! That will save your bacon. If you are unsure about corner speed, ALWAYS err on the side of too slow. Once you can see through the corners, THEN accelerate out. Doing this one thing will make a tremendous difference in your comfort level. Do some googling on Delayed Apexing. That is another invaluable technique, one I use ALL the time.
The curves there are much better than what we have here. Remember that where you are looking is crucial. ALWAYS keep your head and eyes up, looking ahead through the corners.
If you are in a corner and feel like you are going to fast, DO NOT STAND UP THE BIKE! DO NOT CHOP THE THROTTLE! Just start chanting, out loud if you like, "Look, lean believe!" Stay ON the gas, at least holding it constant, and lean into the corner. 99% of the time, the bike WILL be able to make the corner.
If you feel tense, flap your elbows like you are doing the Funky Chicken and take some long deep breaths. This will really help you relax. You want to have a light grip on the bars, not a death grip.
On past rides, I had someone else lead the fast guys and I stayed back to lead the slower and less experienced riders. Over the course of a 400 mile day, we arrived back at the hotel about an hour after they did, and that only because we jusssst missed the ferry on Hwy 125 and had to wait about 40 minutes for it to come back for us.
Riding in Arkansas is all about rythm. Don't slam on the brakes going into corners and then blast out like a missle. Smooth is the name of the game. Get yourself a comfortable pace and go with it. If you find your heart beat creeping up or your adrenaline pumping, SLOW DOWN! If you have any "scare" moments, SLOW DOWN! I cannot stress this enough.
As for the distance thing, you will be sore. If you are not used to riding 300-400 miles a day, by the end of the day your concentration will begin to drop off. Your muscles will be tired. It really helps to do some stretching. Whenever the group stops, spend just a minute or two stretching and walking around. Be sure to stay hydrated as well, not just with water, but something like Gatorade. This makes a BIG difference towards the end of the day. It does not matter if it is not hot out either. A camel-bak is good because it lets you drink small amounts frequently as opposed to large amounts all at once. Chugging large amounts at a stop does not good because your body cannot absorb it all. I think the average person can take in about 8 oz every 15-20 minutes. Drink more than that and it just passes through the system and makes you have to whiz a lot. Lastly, I would take 800mg of Ibuprofen with dinner the night BEFORE the first day of riding, and again at breakfast the day of the ride. Then at lunch and dinner, and the remaining days of riding, I would take a normal dose (400mg). This really helps with the sore joints and stiffness.
For you guys that are the ride leaders. As the day wears on, people will be getting tired and their concentration will fall off. I see this on every ride. People will start making stupid mistakes. You need to drop the pace down accordingly. Even a drop of about 5mph can make a big difference. Watch for this!