helloo.. Mariano From NJ here. they say nothing would prepare you to go up and down hills with a heavy load on your back than actually going up and down hills with a heavy load on your back.. don't worry about it.. you'll be fine.
i has not fit at all.. and had never done any hiking before i hiked the hilly NJ section of the AT. i averaged about 14 - 15 miles a day..(it did hurt a bit) but i had a great time and i fell in love with the trail.. by now i've finished ny and ct and i'm going for ma and pa next year.
don't think too much about it ... it beats the purpose of going hiking ..
happy trails!