If your serious about a 60-80 mile hike, your going to need a large volume bag, extra shoes, cloths and towels take up the most space. These days I prefer day bags that keep air between your back and the pack, so you don’t stick to the bag or get the shivers when you take it off and your sweating.
Whatever bag you choose, absolutely has to have a belt strap to take the load off your shoulders.
Lots of people like the camel back function on the new bags and surplus combat kit. I prefer to wear my water on my belt, If a bear or heaven forbid a moose puts the run on me and I have to climb a tree, I want to be able to drop the backpack and not worry about getting thirsty if the predator is patient.
I know a lot of people like the alice packs and other surplus gear. I do to, I just don’t like the attention they draw.
Another good option is a surplus body armor rig, tons of pockets, even distribution of load, and it screams do not **** with.
I picked one up for 50 bucks here the other month, doesn’t hold as much as the bag does, but you can wear a smaller bag on your back along with it to compensate.
Never be afraid to do the righteous thing, nothing righteous is ever easy.