The marathon training approach has to be used as a vehicle for concentration, not just grunt endurance. You are striving to hold one point of focus, breathing and focusing like a laser beam. I've noticed that with older, advanced black belts, older karateka, the longer they practice, the deeper their concentration gets. It's tough, because even if you think you're concentrating strongly, your opponent may have even stronger concentration. Everyone knows that to acquire anything in the martial arts, you must practice. .
Concentration is just the same.
In order to acquire and build concentration, you've got to practice it. First of all, you have to recognize that you do have a fear. Then you've got to identify what it is, where it came from where you acquired the fear. Then you might even consider if there are any benef its in holding on to this fear. Finally, you've got to understand what benefits you are going to reap from getting rid of the fear.