It's fully possible to keep the finer details of your workouts a secret, even with a published schedule.
Your workouts do need to be published in order for your heat schedule to publish, however, you can keep the details of the workout a secret by leaving the content of the published workouts blank. In that instance, you'd publish your workouts with the correct time domains but leave the content field blank until you're ready to reveal details. Athletes will be able to gather from the schedule how long the workouts are, but won't know details on rep schemes or movements unless you include them in the workout content.
Check out this article for more on setting up your workout scoring content and policies.