Considering age and the fact that you use your knees pretty hard (65 lb pack and all), I'd say it might be early signs of knee osteoarthritis that is being exacerbated by your workout regimen. Sometimes symptoms can flare up for who knows what reason, but most cases are brought on by some mechanical stimulus/injury. Training, and resistance-type training, is generally good for your knees if you've got those aches and pains that seem like early OA, but too much or done the wrong way, anything can do more harm than good.
Talk to a Ortho doc or PT/AT about your symptoms and what you should/should not be doing. I presume that you're not willing to stop hauling around the 65 lb pack (heck, I wouldn't), but you can modify other things and train more effectively so that you get more years out of your knees. Key thing here is to get a really good doc or PT/AT who will sit down and work w/ you...and, quite frankly, is knowledgeable. There's a lot of unsubstantiated stuff being recommended by Orthos/PTs/ATs, and some stuff that is downright quackery. It's difficult to sort through it if you don't have medical training, so shop around and use common sense.