Recently, my son was told that our 10-year-old mixed breed bitch (terrier, border collie, etc.), weighing a bit less than 12 K (approx. 26 lbs.) has been over-exercised and thus having her life expectancy endangered! I was totally floored by this, since I have always let her lead the walks (when I could) and most times curtailing them myself--Spice seemingly willing to go on forever quite happily. She has had a bout with some sort of virus that attacked her spine and was on heavy doses of antibiotics, rest, pain reliever, etc. She seemed to recover from this quite well and had returned to "normal" life.
Her present vet has given her an extremely thorough exam including many lab tests and x-rays. He has her on a special geriatric diet, minimal exercise, medications of all sorts and is concerned about: liver size, possible heart and kidney disease, and all this since I left the dog in my son's care for the next couple months. I am in total shock! What I really need to know is: does a dog of her parentage and size really have to be exercise restricted?
I need to preface my answer by letting you know I am not a vet, so I tend to follow my vet's advice whenever I need it. Yes, a dog with the kind of breeding yours has should naturally be a high energy dog, but the fact that your pet has been through such a traumatic spinal illness, is under-weight, and appears to be having trouble with other major internal organs would signal that some kind of a slow down is in order.
I do think you might find it reassuring to get a second opinion from another vet. Be sure to let him/her know all the current medications your dog is taking, and her history of recent illness. If this second opinion is the same as your regular vet's, I'd go along with the diagnosis and continue limiting the exercise to a minimum as directed. You can make her exercise less stressful to her system by sticking to level terrain and keeping the walk at a leisurely pace, instead of a romp. This way she can still receive the beneficial mental stimulation she gets from her walks. My best wishes for her recovery.
Thanks for your question,