>>2836406
With regulating back(height). №7 on pic.
With good material like cordura, with good stitching.
Belt must fit good and be strong (better thick belt).

Don't take for serious hicking routes cheap noname backpacks.
It must fit good.
I love surplus, I took Tasmanian Tiger 75L with carbone and aluminium frames for 40 km (25 miles).
Also good is Tatonka, Deuter etc.

No matter how waterproof your backpack, it doesn't save you from falling in river on ford and heavy long rain. Put all in small (5-10L) strong plastic bags (vacuuming also good).