Don't apologize for inexperience, especially now that you have ridden a 400k. That's the classic randonneuring distance. Longer rides are just a 400k plus some other rides. 1200k == 400k+300k+300k+200k. See? Easy.

that might just be a symptom of dehydration. Any other signs? I'm almost always a little dehydrated on brevets.

I think eating dry things on a long ride gives most people problems. And then it takes a lot of chewing to swallow, which is very inconvenient, if not gag-inducing. That's why bananas were so treasured back in the day. I like to eat things that are at least a little moist. A counter-example is payday bars. They work really well, but they are dry. Best eaten if you keep them in a pocket for a while.

I have a lot of problems eating things too much and then not wanting them at all. Cliff bars comes to mind. maybe if I was bonking. I made it through most of a 1200k on chocolate milk and reesee's cups, now they leave me cold. I should probably try that again though, they are very effective for me.

I need a better front bag so I can carry food. It's on the todo list.