the pemi loop is 30mi, 49km right? typically that would mean between 3+ days of hiking, in that case you'd probably be looking at a much larger pack than the ones you described

i agree with the other comment saying that 60L is a good place to start - however, the size of your pack will depend on how many days you're out for, what kind of food you eat, what gear you bring, etc.

imo, you should gather all the things that you'd be bringing on such a hike, (like shelter, sleepihg system, food, etc), figure out how much they weigh and how big they are, and then choose an appropriate pack - don't impulsively purchase a pack before considering what will actually be going into it