The inner self is a magical place, it's like that best friend you had as a child, you laughed, talked and played together the friend no one knew about but you, the best friend that comforted you, when you were in pain, and only you knew they were there. What does this have to do with "Mental Workout"? Children are closest to Spirit than any one. Children don't question, they just participate, and they know that there is a part of them that is unseen, and that their young knowing is misunderstood, yet they interact with it daily regardless of what they're told; or how often they are scolded not to talk to themselves. :) I believe we should enter our "Mental Workout" in the same way playfully with childlike abandon anticipating and believing that anything is possible. Opening our minds to the idea that what we are doing is limitless and opens up all kinds of possibilities not only for ourselves, but others.
I feel that inner work should be fun, and can be very much like that childhood experience when you created whatever you wanted, all from within. You gave no thought to the outcome you just believed. I believe mental imagery, meditation, and what Dacher calls "Mental Workout" is the same thing. The inner self the part of us we have neglected and as a result must retrain by doing "Mental Workouts".
When Dacher suggest that we do something called "Mental Workout", what he is suggesting is that we do mind exercises; I would compare to calisthenics, naturally expanding our mind by using only our mind. And doing this training requires discipline by setting a specific time, and creating a sacred space in which to do your practice. By doing this you can expand your consciousness towards enlightenment.
Dacher says: A time set aside each day for exercising the mind through meditative contemplation with the purpose of learning to use more of our minds capacity". This is how we arrive at the place of "Olympian mental fitness". The idea is to stabilize the mind thus entering into a higher state of consciousness.
I would call it "Fine-Tuning" the mind. Desiring to be conscious is personal, wanting to be more aware is a positive. We can utilize whatever tools work best for us; however, Dacher's model is a great start. Happy meditating.
Dacher, E.S. (2006). Integral Health: The path to Human Flourishing. Laguna Beach, Ca. Basic Health Publications.