Nike had been showing signs of age for some time now, and hasn't been able to run with me since shortly after the twins were born.
But then again, I wasn't about to let him starve himself to death, either. It was one of the hardest decisions I've ever made, but luckily, the vet (dad of my old friend Dagny) said he would have done the same thing if it were his dog, so I have some consolation.
Nike was the best dog. He was well-behaved and hardly ever did anything to get in trouble (except the occasional taking food from babies, and they practically have it to him so it's not really his fault), he would run with me for hours at my pace, and he let my kids climb all over him.