I'd recommend heading out to an REI and getting fitted there. Also you will want to try some packs and put some load in them. Walk around the store for a while and try several out.

YMMV at REI but i've found that there are a lot of people working there who do actually go outside. Ask the sales person if they hike and how much, if you don't like the answer ask someone else there.

Don't buy on impulse.