Update 2. This is now achievable doing repeat workouts. When the challenge first appeared live, progress would stall at 25%.
You will receive 4 stamps as a reward for completing this challenge.
Update 1, as provided by MathGuy42...
"If you want to get this very quickly, you can start a workout, then Fast Forward to the end. Hit your Menu button, then select the super Fast Forward (two triangles pointing right and a vertical bar). You'll go to the last few seconds of the workout, but it will count.
Also, you usually don't get the stamps until you complete another workout (which can be fast forwarded through as well)."
Also, if you're going to downvote this, and you actually have a good reason, please post why so I can update with any relevant information. Thanks.