My first 100k with the Seattle Randos, I crossed the finish with 1 minute to spare!
Unlike a supported metric or century, a rando ride is a totally different beast; no SAG wagons if you break something, no sun-shaded rest stops stocked with Clif Bars, cookies, bananas and cold water, no Dan Henrys to tell you where to go... and the clock ticking away the entire time.
13.6 isn't a bad speed to keep. On my rando rides, I'm usually around a 14.5 average. Heck, one of our local legends (Kent Peterson) wrote a blog entry called Life at Twelve Miles per Hour; all about slowing down, enjoying the route, and taking your time. Considering he's riding from Issaquah, WA to Banff, Alberta, CAN to race the Tour Divide, and then he's riding back home from Mexico... he might just know a little bit about long distance riding.
The Camelback is probably a good idea if you're not keen on warm water. Up here, it's not so much of a problem. Down there, it's a heck of a lot hotter.
Ditching the Carradice might be an option for now, but don't count it out for long rides in the future. I have a 12L Berthoud up front which stays on for all my rides, and I typically leave my Pendle on the back, too; even if there's nothing in it. Yes, it's easier to keep snacks and stuff in the handlebar bag for quick access while riding. Plus, it's nice to have the map case on top of most h'bar bags. But if/when you start going for 300k+ distances, the extra space afforded by the second bag is nice. You can balance your load front and back, which is good if you have to carry a bunch of extra clothes for night-time temperatures, or if you have a course with long stretches between controls, and you need to stash a couple spare litres of water and 50 miles worth of food.
Getting the pattern down for your eating habits can be tricky with randonneuring, since the controls are less frequent than on a club/charity ride, and you're usually left to fend for yourself. Sometimes, getting queasy is part of the game. I know many randos, and I've been one myself, who have fallen prey to a rebellious stomach and had it end our ride, regardless of how we tried to press on.
Our club president has a t-shirt that says "You're not a real cyclist until you've thrown up in a French roadside ditch."
Check out some of the advice on the Rando/LD/Endurance forum.