Well I'm only eighteen and have been backpacking forever, and at the end of a day even I am sore everywhere. Particuarly the knees. I don't like taking meds, so I use Tiger Balm. It's a salve of sorts and it comes in a small glass container, very lightweight so you dont notice it in your pack. It's strong stuff, a little goes a long way so it'll last you an entire trip. Rub it on any sore muscles or joints and it feels great. you can buy it at drugstores. Also try and eat as much iron rich foods as possible. It's tough but iron raises your hemoglobin level which helps bring more oxygen to your muscles. This will help prevent fatigue after a few days on the trail.