Even though we like to think we are no longer mortal while running gear the sad truth is.. we are. Over training is surely an issue and the most important thing you can do for yourself is to listen to what your body is telling you. It will ALWAYS tell you when to slow down. If you have reached a plateau then you need to change things up, change your training, take an extra day of rest, change your diet. Over training will kill your gains just as fast as a poor diet.