You need a backpack that will carry your gear safely, while at the same time taking most of the stress off your shoulders. Fitness level is also a big factor. Ideally, the backpack should have separate compartments for gear and other necessities so that you're not rummaging through the pack to find something, provisions for carrying water and a tripod. Anticipating your needs, and thereby minimizing the amount of your gear, also helps.
Regarding backpacks, "most" camera backpacks are usually heavy, which is not a good starting point. Take the aforementioned Tamrac Expedition 7...I originally bought one of these a while back, so I'm able to comment on it from experience...it's freaking heavy when empty...close to 7 pounds. The outer pockets are pretty much useless IMO, and no way to carry water bottles without having to buy accessories to clip onto the outside. My Tamrac has been relegated to the attic...one of those "buy before you try" decisions that I learned from. It'll hold a ton of gear, but it just didn't work for me.
Some camera backpacks do a decent job...the Lowepro Rover AW II, which I now use quite a bit (and its waterproof sibling, the Dryzone Rover for when I'm in or near water). The Rover is surprisingly light...only about three pounds give or take. Has a bottom compartment for your gear, and a roomy upper compartment to carry the necessities (light jacket, first aid kit, food/snacks, penlight, etc.), as well as two outer pockets for water bottles (each will hold a 1-liter Nalgene bottle), and a tripod carrier.
There's also "real" hiking backpacks that could be modified with a camera insert...these are better designed to distribute the load, are fairly light themselves, and usually have an internal frame to help things out. A 30-liter pack with several external lashing points should do well for arduous day hikes.
Last but not least, if you're going to do any type of hiking, a lighter tripod is a must. If you don't have a carbon fiber tripod, you should consider one. These will typically weigh in at half the weight of "traditional" tripods. They cost a bit more, but the weight savings are well worth it (and your back will appreciate it).
Regardless of age or fitness level, the key is to lighten the load as much as possible, while still allowing yourself the gear choices to come away with the ideal shot. Start with a good backpack choice, and work your way from there. Cut a few ounces here, and a few ounces there, and the weight savings will start to add up quickly.