Interesting. I have a terrier (~15 lbs). He keeps up with me for 45 minutes and seems barely tired. But if the temperature is above 75 degrees F or so, I can easily run farther and faster than he can. Right from the start I can tell he's less enthused and wants to run slower (which we do, or he stays home; I don't want to bring him anywhere near heat exhaustion).
I was a little concerned at first with my dog, running long distances, but we worked up from 15 or so miles per week to 50, and he seemed fine with it. But I was running in the early morning at the coolest time of day, so I'm sure that helped him.
