The treadmill

You give Tracy a moment to recover from the ordeal of having her asshole stretched out like it never had before whilst performing her squats. She continues staring at her own ass in the mirror in amazement at just how wide the gape is, she's probably seen bigger in all that porn she's been watching but still, she'd never seen anything like this relating to her own body. Her legs seem to have stopped trembling as she relaxes after the workout.

"Alright slut, time for your final exercise of this session."

Tracy finally averts her attention from her own asshole and turns her gaze to you and then to the treadmill. The two of you move over to it and she starts looking over what you've prepared, eyes wide with astonishment. Wrapped around the handles are some nipple clamps attached to each other by a reasonably long chain.

"So as you can see I've slightly modified the treadmill so that if you slow down you'll feel a painful tug on your nipples. This way you're incentivised to keep a good pace whilst you run, or else face a harsh punishment. Why don't you climb on up."

"Yes sir." Tracy looks at the setup with contempt and self doubt, wondering how much you expect from her and if she'll be able to make it through without suffering too much pain, although from her past behaviour perhaps the masochistic slut would prefer some pain in her tits. Nonetheless, the clamps would be attached the whole way through and applying a light amount of pain, hopefully enough to satiate this sluts hunger for punishment.

She climbs up onto the treatmill and you grab each of the clamps, attaching them one by one to the areola of her nipples so that they'll be pinching her tits tightly. Thankfully none of the massive load that was still covering her tits were quite on the areas where you attached the clamps, otherwise your cum might have caused them to slip off.

"Ouch, that hurts."

You give her a playful smack on the ass and say "don't pretend you don't enjoy it slut." She smiles and avoids eye contact out of embarassment.

With Tracy all set up in position, you set the speed on the console to 12km/h, something you know she should be capable of maintaining, considering she's already quite fit. She works her way up to a comfortable jog, but not quite fast enough as she falls slightly back on the treadmill and the chains go taut, causing the clamps to painfully tug on her nipples.

"Ow fuck." She says as the clamps pull her tits forward slightly, strongly encouraging her to speed up. She immediately does so and maintains a steady pace so that she's as far forward on the treadmill as she can be, giving the chains some slack and relieving the pain in her tits. Her tits are bouncing from all the running causing the occasional tug on her nipples and a sharp breath from Tracy.

"Alright slut, I'm gonna leave you here for 40 minutes while I go finish up some work. Make sure to keep up your speed, for your own sake."

Tracy nods in affirmation, not wanting to interrupt her controlled breathing to speak. Satisfied that your slut will be improving her fitness, and therefore her performance whilst fucking, you leave her to her workout and go to continue your work.