Going the distance with a dog

IHT, with this post about her goals and increasing her running distance with her dog, has gotten me to really think about how far I plan to run with Cosmo. W and I have discussed it a few times while running but as we start to really increase our distance, it is something I seriously have to consider. So far, the boy is the picture of health - his feet and pads (a little obsession of mine as we run on asphalt sometimes) are perfect, so says the vet. His heart is strong and solid. After runs, he is tired but he is never stiff or slow moving (unlike me!).

The problem is he loves to run and would probably run himself into the ground before stopping. It goes without saying that is unacceptable. I know he can do 5/6 miles if the weather is cool enough. Can he go longer? Can he do 8 or 9 miles or even longer? I know there is a distance where humans have to start taking in calories to keep going. I assume the same would be true for Momo.

Of course, internet searches have turned up almost nil in the information department - just lists of distress signs you should watch for. Yeah, I don't want to wait to see my dog distressed before I stop him. Just how far can Cosmo go?