I feel the same way about my cat. Wish she would never have to die.

Thankfully, she's 7, we thought, and vet has confirmed, that she's physically as healthy and active as a 3 year old cat, and the vet says as long as nothing major happens, we can probably keep her going for a long time. She gets more exercise than an indoor cat usually gets (thanks to her randomly running around the house for no apparent reason) and she's a good weight, so she's in very good condition. Vet said the way she's going, she could live until she's 20 or older.
I lost a cat when he was 10 from kidney failure, so I can imagine what it must be like for owners of large dogs. His life was way too short.