The life expectancy of Chinese geese is said to be about 20 years. I know parrots can live to over 50 years, hens up to 15 or in some cases 20 years, so 26 seems to me to be a very good age for your goose.
While she may still be very healthy, have you considered that she may have arthritis in her leg joints? You could look at cod liver oil to help her joints and if you have an avian vet nearby, ask them if glucosamine is alright for them. I know green lipped mussel shell is available for dogs over here for arthritis, so perhaps the vet could recommend something to keep your goose pain free and help her mobility.
All the best to you and your goose. It sounds as if she is very much a family pet and you have looked after her very well.