I've had this fantasy for years. Just can't convince my sometimes sub wife
I've often had the fantasy of having that type of agreement, where bdsm was tied into the physical training.
I think there are some pictures on Dee's website (luvbright or something like that) where she is in a chasity belt and the nipple clamps are attached to the tredmill, and she had to go so long before she was let off the treadmill.

amazing how a good crop to the rear can keep them going for just a little longer