I'm sure Banditsocks2 will pop up in this thread... but she's got two little ones (I think 5 and 6lbs) and they can go on forever, there's a circle in the neighborhood that's like 4-5 miles and they do it fine.
Jackson's 15lbs and I think he tires out before they do...
haha, but no he loves it too. He can walk a long time. He can pretty much go all day if necessary, whether it be swimming, running, whatever.