A calm mind, a toned bod, something to brag about: The benefits of physical fitness are bountiful and numerous in their great quantity, etc. Just as important, however, and far less paid-attention-to, are the benefits of knowing when to give up. Knowing when to roll over on your cat belly. Knowing when your owners are filming you for YouTube, and knowing how to ham it up real good.
That seems like enough sit-ups to me, and I applaud your reasonable attitude with regard to exercise. Good cat!