Re: I need a decent rucksack ASAP.
Gotta agree with chrisotpherm and gusto on this one. Large ALICE pack is the way to go but like gusto said add some extra padding if you are going to be covering long distances. Like MM said check the army surplus in your area. If you can't find one there plenty of places on line sell them. You should be able to grab one up for about 50 bucks. I actually use a small ALICE pack without the frame as my gym bag.
There is no hunting like the hunting of man, and those who have hunted armed men long enough and liked it, never care for anything else thereafter.