I doubt that military look-alike backpacks are significantly more durable than many of the better civilian backpacks commonly available. Several of the brands mentioned in earlier posts (Gregory, Kifaru) have very good reputations.

What is most critical in a backpack is fit; most current brands are adjustable. I would recommend going to a good outdoor store that offers fitting service and try on a loaded backpack and fine tune the pack while in the store. REI would be my choice, although they are not the only outfit with this capability. All of their packs are good quality, including their house brand. I'll bet you would be very happy with an REI Traverse 75 (or equivalent) for your contemplated usage.

Some people (especially ultra-light backpackers)can do a seven day trip with a 28 liter backpack; others will need a 75 liter backpack for a minimal three day excursion. I'll bet you will wind up with a daypack/overnight pack of about 28-30L and a larger pack, 65-80L for more extended trips.

I probably have about a dozen backpacks of varying size and age; some are more than thirty years old. I am impresses with the quality of the better brands available today. The only failure I have had was on a Maxpedition pack (military style), which was repaired easily.

Again, I would go with REI; they have an excellent warranty and return policy. I have been a satisfied member, buying a lot of their stuff for over fifty years.