That's amazing Q... (the cat's age, not your 'performing' in the garden possibility). Our cat died about 3 months ago at the age of 14 and the vet allowed as how she was quite advanced in age, cat-wise.
I concur with leelapops suggestion. Our cat (actually, Mrs. C's feline) Tia would use her regular litter box quite well for weeks on end and then decide she liked the deep-pile throw rug in our bedroom... much to my consternation when stepping in the wet spot before day-break. At any rate, we placed two litter boxes in the usual place... the cat appeared to have an aversion to using the one litter box more than two or three times. She also would use another throw rug at one of the entrance doors to the house at which position we placed yet another litter tray. This seemed to solve the problem, with an occasional excursion on her part requiring a movable litter box on our part. (Actually, I, being the non-cat fancier, suggested a more direct approach, that being a well placed cowboy boot tip rocket launch out the open door, which was nixed by the lady of the house in the equivalent of a well played hemidemisemiquaver)... Ah, well the white, long haired Persian of surprisingly corybantic persuasion has gone to a far better place (according to Mrs. C at least)... Best of luck with the elderly gent!