Although I agree w/"Pit Chick" that BC's are high energy and bred to "herd".....as a guardian of an Aus. Cattle Dog who tends to get "over heated" easily on sunny days, dogs that herd live stock move in "spurts" NOT to "jog mile after mile non-stop" .
Furthermore, BC's have quite a fairly thick fur coat--and so that is a factor to consider.
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Dogs will try to keep pace w/their person out of love and excitement of being outside, even if they are lagging behind and tired....I see this a lot. I'm sorry if I'm sounding negative--I don't mean to....
Pets need exercise--just don't over do a good thing.

I think the BEST rule of thumb should be "Is the dog comfortable and enjoying itself the ENTIRE TIME??? Put the dogs needs and comfort level above YOUR own desires to get a great work out