To the original poster - you gave this dog a good life. A dog that has a job of protecting those who he loves, of keeping them company, that is a happy dog.
I don't suppose anyone wiil ever invent a way for our dogs to tell us, but I think if they could they would indeed tell us this:
Thank you. You gave me a good life. I am happy that I was able to be with a good master who loved me. Now I hurt, and there is nothing more that can be done for my body, and I know when that time comes, and then, all I want is to be at peace. You can let go of me now, I am going to a peaceful place where I won't feel pain or hurt. My legs will be strong and I will be able to run again.
And when it's time for you to go to that peaceful place, I will be waiting at the gate for you, and I will remember you and wag my tail when I see you. And we will walk together again in that peaceful beautiful place.