I'm just doing this as exercise and preparation for military should I decide to enlist. I don't have a frame for the pack, is that something I should get? When I put a 35 lb plate in there it pokes out because there is no backing for the pack. Heavy solid objects don't seem to sit very well in ALICE packs. Any advice?
Lot's of good advice so far. I once did 100 miles in four days with an ALICE pack. The long and the short of it is that they really aren't all that comfortable. If you customize your pack to the point where it's comfortable, you're defeating (what appears to be) your purpose - preparing yourself to carry weight in an ALICE pack.
A couple other thoughts: Build up to heavier loads gradually to avoid inuring yourself and delaying your preparations. Start with 15 or 20 lbs and gradually increase to 55 - 60 lbs over a period of 12 - 14 weeks. Learn the Airborne Shuffle. If you try to run with a 55 lb pack the way that you run normally, you're going to injure yourself. I made a video of the Airborne Shuffle for another forum. Here you go: https://www.youtube.com/watch?v=QldGPM_Z9-8
As you view the video, keep in mind that my speed in the video is relatively fast. I've built up to the point where I can run 8:15 miles for quite a distance while carrying a pack. It's going to take you a while to get there (and there's really no need to get to that level of speed). You might target a 12-14 minute mile with a 55 lb pack.
If you want it to be like the Marines, get a frame, then break the frame. Break a couple random fasteners. Adjust it so one shoulder strap is always flipped around and digging into your skin. The other shoulder strap should always come loose. Get it wet and leave it in a dark basement for a few weeks to replicate the aroma. Also, make sure it has an obsolete camo pattern that doesn't match any of your other gear.