Please, please consult your veterinarian first. Start with short hikes of just a mile or two and work up. You shouldn't expect your dog to carry a pack considering her age and injuries. Remember that 12 for a dog is the equivalent of 84 for a human!

While the hikes probably have to remain short, at least you'll be giving your dog a good time, which is important for whatever time she has left!

Had you considered dropping her off at the friend's house and then going home for your pack? (That assumes she hasn't seen the pack beforehand!)