Sounds like a lot of fun you guys have for being so outdoorsy! lol
Although at 13lbs and with such athletic owners, I am sure Penny is pretty healthy and strong. However, I would hesitate to make her carry her own weight in food... or a little weight even since during a long hike, you get exhausted no matter what and her knees probably don't need any more weight added to it than her own. Heck, I'd love for someone to carry my backpack and bags around whether or not I ought to be able to carry some weight.
Being free of any kind of burden certainly makes any amount of exertion a little easier. Of course, the humans will have to suffer the extra weight of a bit of dog food and water.
For less strenuous excursions like a trip to the park, I'd bet it'd be nice for your dog to carry a few of their supplies along.
Those backpack thingies for dogs are so cute though I mostly see Outward Hound ones only!