Two bits of advice:
1) As you get more experienced, you'll realise what you must carry, what you should carry, and what you don't need. But as a first ride, err on the side of carrying it if you think it might be useful.
2) Do this mental exercise. Imagine you are riding in the first 4 hours of the ride. What would you carry? Then do that mental exercise for the next 4 hours. And then the next 4 hours. Add them up, remove things you've double counted, and that's pretty much what you need to carry.