I have ridden with a small camelbak and discovered that I could only fill it about 2/3 full of water or it would start to be painful after about 30 minutes of riding. Even 2/3 full of water or less (because I drank the water), it gave me some serious pain after a while.
I have ridden with backpacks as well. For my 6.5 km commute in Winnipeg, my backpack was fine. But for my 30 km commute here, my backpack was NOT fine. I used it once and that was it.
Do a test, just grab any old backpack you have lying around your place, fill it with whatever you want to carry in the backpack, and do a ride of the distance you think you'll usually cover in a day. That should give you a pretty good indication of what carrying something like that on your back feels like. Even better would be to go out the very next day and repeat the same experiment. If it felt all right on the first day, it might feel quite different on the second day.