She didn’t consider herself an athlete. Not like Rainbow Dash or Applejack. Still, she was no slouch. A canter with Twilight every morning for stamina, and the pilates classes with Fluttershy after their weekly spa meetings. Between those and running a shop by herself, she was in great shape.
She probably wouldn’t win gold, had she any inclination to join a running of the leaves, but she’d get close.
Fact of the matter was that she had a semi-secret stamina building exercise. In fact, the six of them were all amazingly fit.
You had to be to survive Pinkie’s parties.