Welcome to ETS! You've already received a lot of good advice, but I'll add my $.02 worth.

As others have noted, the first thing to do is decide what you want to carry, and where you plan to carry it. Military packs are designed to meet the needs of military personnel in combat. Yes, they are built (overbuilt?) very tough, but that doesn't necessarily make them better for your intended use. Soldiers in combat often carry very heavy loads (50-70 lbs, or even more). All this extra weight is often bulky or oddly shaped items, sometimes with sharp edges. Think extra ammo for personal weapon, extra radio batteries, night vision gear and batteries, smoke flares, mortar rounds or belts of ammo, breaching gear, entrenching tools, etc etc. Combat will be very abusive on gear. Think crawling through broken glass and rubble, hanging packs on the outside of vehicles, crawling through (not around) brush or rock slides, etc etc. How likely are you to be doing that?

The military buys gear in huge lots, and tends towards a "one size fits all". If you aren't that size, the fit might be less than optimal. Also, these days most soldiers will be wearing body armor, and military pack straps are designed with that in mind. Again, unless you wear body armor, the shoulder straps might not be the optimal fit for you.

Military gear often tends to be very heavy, relative to civilian packs. Military packs have lots of Velcro and other places to attach other gear. That looks cool, but adds weight. And those packs are designed as part of a system, so the attachment points mate with other pieces of gear in that system. Unless you have (or need?) that other gear, all that Velcro just adds useless weight. A pound on your back is a pound on your back, whether it is food you can eat, or useless (to you) extra straps and Velcro. A weight that feels comfortable in the store or on the first mile of trail, can feel crushing after many hours and miles.

As others have noted, I would strongly advise going to a good retailer such as REI who specialized in backpacking gear. Talk to the sales folks, get fitted for a pack. They can also advise you on some nice pleasant easy intro backpack trips in your area.

That depends. What sort of terrain and climate? How long a hike each day? Alone or with others to share group gear? If you haven't backpacked before, I would strongly advise starting with a more modest trip. A short hike followed by a one night campout is ideal for starters. This will give you a chance to evaluate what you need (and more importantly don't need), and get used to your pack and other gear. Any problems that arise (and some probably will), won't be that big a deal if you are only out for one night. I know far too many people who did a long trip for their first backpack, had a miserable time, and never wanted to do it again.