You can train too hard. Although running more miles generally means better fitness, it is possible to run too much before your body is ready. It is one of the most frustrating aspects of running because people who suffer from over-training are usually the ones who are most dedicated to the sport. Perhaps the most common symptom of over-training includes a general feeling of tiredness during runs that does not go away after taking a day off. Other symptoms include lack of desire to run, continuing soreness in muscles and/or joints, and a loss of appetite.
The general test for whether a you are suffering from over-training is to take your heart rate when you get up in the morning. If your heart is 10 beats per minute more than normal, there is cause for concern.
The treatment for over-training is obvious: rest. It is important to take at least a whole week (preferably two) completely off from any exercise. I understand all too well the difficult of taking time off, but if you continue to run, things will only get worse and you could develop more serious problems than simple fatigue.