(wipes tears from eyes) had a similar situation with my dog that I grew uo with- collie, we got him when I just started school and saw me through to work!
Like Fatty, Ben went blind(gradually), deaf(gradually) he always used to lie behind the front door and would jump up to greet you, or bark at strangers, as time wore on you had to push the door, and dog to get in!
He loved to go out, and would do errands o n his own, but as he got on life became harder and more tiring. His legs got heavier, and hips a bit stiffer, untill he could only make it round the block.
A quiet trip to the beach-his favourite, and a short walk through the park to the vets, with treats on the way, and we said goodbye after 15 years.
Im sure you would want to be kept going for someone else’s plesure/lack of guilt.
Sorry if it sounds a bit twee. Good luck