Likely, what you're seeing is age-related arthritis...at the age of 12 there is bound to be some arthritis. Changes can occur in the joints or along the spine, and they can certainly cause the symptoms you're describing. And at the age of 12, a dog will definitely have less energy.
Continue to keep her weight in check, and take her on more frequent shorter walks instead of long trecks. Once capsule of glucosamine daily might not be enough--our 60-pound dogs are on two capsules as a maintenance dose, and some dogs require more.
If she'll tolerate them, booties to protect her hind feet might be a good idea.
Have you had any geriatric blood panels done recently? It'll tell you a lot about her general health. Also, if there are ticks in your area, screening for tick-borne disease might be a good idea--our dogs often get stiff in the joints when they're fighting anaplasmosis or Lyme's.
Good luck with your girl. She sounds like a lucky girl--well-loved and well-cared for!