OK, in my opinion.
at 80 miles its going to take a while to get home.figure out what you want to carry to get home, then size the pack to fit your items,if you get an extra large pack you'll windup over packing,then you will toss it out on the way home.
so a big pack or resupply on the way home allowing for a lighter pack.(can you stash some supplies somewhere)
can you carry a bike? that will increase you range per day.
do you want military look or hiking look?
i went for a hiking look trying to not stand out, a lot of people here carry hiking packs.
for hiking packs take a look at the at the backpacking magazines for reviews.check online at rei.com backcountry.com campmour.com ect and read the reviews available at the library
you'll be happy with a durable pack versus a lightweight.especially if you move it around a lot. multi day pack depending how you load.are you carrying a weapon?
i have an rei pack and a couple of kelty packs, osprey and arcteryix are also good.
and if possible try it on before you buy.Have Fun.