Less food, more aerobic exercise, after a comprehensive geriatric wellness examination (blood work/dental) by your vet , which she should have begun getting every six months by age eight, to determine whether she is healthy enough to proceed with any aerobic exercise, which should be introduced gradually over a period of months, if she IS Healthy enough, this is in addition to her routine daily walk/elimination times.
Tinned food has very little nutritional value as it's 85% water. "boneless, skinned chicken or white fish" ALONE Is not a nutritionally complete/balanced diet. Properly fed, as well as proper exercise, walking is not considered exercise, it's for elimination only, may take three times longer to lose any weight, than it did to put it on. It's never healthy to gain or lose fast, slowly/gradually is always recommended.
Did the vet do comprehensive blood work, including a complete thyroid panel?
Feed on a set schedule, twice daily, am/pm, no free feeding, no human foods, scraps, treats, nothing other than water that should be available free choice 24/7, split the total daily amount of food into those two meals, the total amount should be measured out precisely according to body condition/activity, adjusting as needed, while monitoring all weight changes on the routine basis.
Obviously, if you haven't been providing even the minimal daily walk/elimination time 3-4 times daily, 30-40 minutes each time, begin there gradually accustoming her to that schedule over a period of months, before you even consider introducing aerobic exercise, then gradually work her up to 3-4 times daily, 20-30 minutes each time. Swimming is excellent exercise, and easier on the joints, does she have arthritis, how severe, is she on pain meds? Metacam may cause liver issues. Gabapentin maybe a better choice for pain, but discus this with your vet who would know your dog better than anyone on YA.
At her age, I'd suggest at least, 6-9 months of working her up to the walking/elimination times, as suggested and keeping your vet posted as to her progress, before even beginning the gradual introduction to aerobic exercise.
Discuss all options with the vet, possible treatments, prognosis, as well as quality of life, then put your feelings aside and do what is best for the dog accordingly. Depending on how over weight she is it may take 18 months to a couple years to lose any significant amount.
Good Luck to both of you.