Just reiterating what some others have said and throwing some personal opinion in... A 35L pack should be sufficient. You should easily be able to fit enough for a 3 day hike in there. If it were me, I would take 2.5 gallons of water, a small jar of peanut butter, a smaller jar of jelly, some bread, 2 dehydrated dinners(4 servings) or a lot of pasta and some butter, 6 packs of oatmeal, some cliff bars and some better tasting snacks, my camping mug, a spoon, a knife, my medium sized first aid kit(with bandages, aspirin, neosporin, gauze and some tape), a very small camp stove if you aren't allowed to open burn, 2 lighters, a sleeping bag, lightweight long sleeved shirt and pants, thermal long underwear, a tarp and hammock, paracord, a flashlight and extra batteries, playing cards, a notepad and pencil.
2.5 gallons of water alone weighs 20 pounds, keep this in mind, but also that you don't need to carry as much water on the way out if there's a source available and you know how much water you used on the way in, just don't cut it too close. The most important things IMHO for a long hike or camping trip are:
Having everything you NEED
Ensuring the people you are with have everything they NEED
Keeping the pace manageable for everyone
Having fun
The kind of people you are going with and the pace they might try to set is sort of out of your control. Especially if your pack seems heavy to you before you go out, just make sure that you aren't going to be struggling to keep up after the first few miles. Keep in mind that when hiking, the slowest should be leading.