In reply to stuartgmilton:
Most rucksacks now are strong enough to survive a three day walk.
I suggest you get a big holdall. Place all the stuff you will take on your walk in the holdall (including food, water, fuel for your stove, etc - the whole shebang).
Go to your local outdoor shop when they will be quiet (early Saturday, or during the week but not at lunchtime).
Explain what you want.
Try lots of rucksacks to see if they will hold your stuff. I find that different manufacturer's 50 litre rucksacks hold different amounts of stuff. Think also about how easy it is to get everything in, and how easy it is to get access to stuff once inside. Think about striking camp in the rain - you want everything in with minimal fuss.
Once you have found rucksacks that wil take your stuff, try them on with the stuff inside. Is the sack comfy? Does it feel right?
Choose from amongst the ones that take all your stuff and that feel right on you.
Consider: side pockets and a zippered lower compartment make it easier to get at your stuff, but add to the weight you are carrying.
Consider: I have a rucksack with a straight side access zip. Never used it.
Consider: my rucksack has a daisy chain. The only thing I have used it for is to hang wet socks out to dry. Added weight, no added benefit.
You usually need to trade weight against convenience against price.
Enjooy!