Optimal performance starts in the mind.  Here is a section from The Mindful Athlete by George Mumford that talks about the concept of flow. 

In the book flow is described as this way: "Flow occurs when both challenges and skills are high and equal to each other."

To perform at the highest level and achieve flow in fitness, athletics and many other things in life Mumford states:

"The real key to high performance and tapping into flow is the ability to direct and channel your strengths and skills fully in the present moment - and that starts in your mind. The flip side is also true.  No matter how strong or skillful you might be, your mind can also impede that talent from being expressed, and it often does so in insidious ways if you don't take care of it.  You've probably heard before, and it's true, that the mind is a muscle.  You need to take care of it through daily practice.  It's that simple and that profound."

The big takeaway from this is train your mind as much if not more than your muscles for optimal success in fitness.