Most students who take the chord-scale approach to learning to improvise never achieve that ability to "play what is in their heads". Few get past the stage of running scales, which does not quite engage the listener.

However that said, learning to play your modes is a way of ear training and technique building. It has been my experience in teaching that if a student cannot play a phrase he most often cannot hear the phrase. I realize this is opposite of most conventional jazz teaching. i.e. "You gotta hear it before you can play it". The reality is that there is no seniority when it comes to your ears telling your hands what to play. Just because one listens to Be-bop 8 hours a day for 20 years does not mean one can play Be-bop.

Developing that ability to "play what you hear" can be an elusive skill. Imagining a musical line is quite an advanced skill, which comes more natural for some. Modes are good things to learn but they are not the vocabulary of music. I encourage you to memorize and perform the language of music. Copy the Masters.

When being interviewed during a recording session a few years ago, John LaPorta was asked the question, ³Now that you have been playing music for over 60 years, what is different about your playing today?" John responded, "I'm finally starting to play what I hear".