This is a field manual for the A.L.I.C.E. pack. If you can, attach the shoulder straps so they do not sit on your shoulders. I had mine so I could fit two fingers between the top of my shoulder and the strap. Its main purpose is to hold the pack against your back, not to support weight. Basically the proper fit for a pack is if you don't feel the load on your shoulders at all, you feel all the load on your waist, When you put it on, lift the load so it sits on your hips, then tighten the gut belt, if it doesn't have a gut belt, GET ONE a padded one at that! You'll thank yourself for it!!!! If the shoulder straps don'th have a sternum strap to hold them together get a set that does. A sternum strap ma Carry it around for 20-30 minutes, anyone can carry an off balance load for a few minutes. First time I loaded one, I loaded it heavy on the right side. Two miles into our trip carrying most of 80 lbs on the right side, I could no longer feel my right arm. I had to stop on the trail and re-pack. It was very annoying for everyone to say the least. Put the heavy items in the bottom lighter stuff on top. Once you pack it, pack it that way every time, it will help you get used to it. I used one for years fishing and it did fine. I did end up with an internal frame pack tho.
An after thought, anything not going inside the pack, but being lashed to it. Make sure its lashed tight! Something such as a bedroll swinging off of the pack will make it as comfortable as the shoulder bag you have been using! I learned that one the hard way as well!
I hope this helps, and I haven't told you a bunch of stuff you already know!