Running: When is it enough?

My 19 year old Spotted Saddle mare is full of energy. She will run and keep up with everyone else when we fly on trails. But sometimes, especially now that she has her longer winter coat, she sweats big time and after rigorous running, she will be breathing heavy and sweating a ton (which I know is normal) BUT she still doesn't mind running and keeping up with everyone! I have to keep her DOWN and even then we do that prancy "please let me go! I wanna go!" walk.

Regardless of the hard running we do, or how long we do it for, she never seems to tire out? Is there a limit? Does she know it or do I have to decide it for her?

I'm not sure how to rule this out for trails and galloping and hacking and what not, but my old instructor used to say "in an hours worth of riding, you should be doing 30 minutes of walking, 20 minutes of trotting, and 10 minutes of cantering"
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Hmm. Any particular reason why?
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