Must be an OTTB thing. Mine did the same thing.
Is she running at the canter, or taking fast trotting strides into it? Your wording sort of confused me...
If the first one, make her stop, canter again, stop, canter again. Make her think that when you canter, it isn't going to be for very long. Pull her into the wall, if you have to. This got mine to slow down and stop running as fast as he pleased.
If the second, if she doesn't pick up the canter in a few trotting steps, make her stop, stand, and try it again. Keep doing it until she gets it. It's frustrating, but it makes them understand.