Hi all, I'm looking to buy a new rucksack, about 25-40L, for hill walking. I want to be able to take it for a short walk but also a full day walk so I need it to be comfortable for this length of time with about 4-6kg in it. I've seen a couple of good looking packs that I fancy but I'm not sure about an adjustable back system? Do I need it? What are the benefits??