That's a tough question.

I would build upon what Glenn said and add the option of Internal vs. External pack types.

The pack must surely fit you. The best thing to do to find a pack that fits you is to go to your local REI and have them size you up. The benefit of this is not only that you are getting expert advice, but also REI has a machine that can heat the hip belt on some packs so that it molds exactly to your hip. This is an option on Osprey packs, which I will discuss later.

As far as size goes, that is very dependent on your interpretation of "long hikes". For some, a long hike is to the front of their neighborhood and back and for others it is a thru-hike of one of the Triple Crown trail(Pacific Crest Trail, Appalachian Trail, Continetial Divide Trail if you are unfamiliar). The blessing and curse of packs is that there are sizes, ranging from a small hip belt pack(100 cubic inches or cu in.) to expedition size packs (5000+ cu in.), for each of these interpretations. If you could describe your hikes a bit more forum members can give you a more personalized recommendation.

As far as the debate between internal and external, almost all hikers now go with internal frame but the decision is still yours. I would recommend you read the guide on the front page of the website as it gives a great comparison between the two types of packs. (Click here for the page)).

As far as brands go, I only have experience with Osprey, and I would be hard pressed to switch my allegiance to any other brand. They offer a manufacturers lifetime warranty with all of their packs that states if ANYTHING goes wrong with your pack they will repair it for you free of charge. I have a Osprey Argon 70 liter 4300-47000 cu in(Click here) and a Osprey Kestral 38 liter 2200-2300 cu in. I use my Argon for most of my back country trips longer than a week and the Kestral for traveling and backpacking (in the non-american sense of traveling through a country only with a backpack). I swear by both of these packs and they have always provided me with exactly what I needed both is convenient, effective accessories as well as durability. I can go into greater detail if you are interested.

Hope this was a help. I'm sure more will chime in with their 2 cents.

All the best.