I just deleted a three page blog entry detailing overtraining and the endurance athlete, because it rambled and didn't cut to the chase. Hopefully the following blog entry doesn't skirt around the issue.
Most of the current information regarding overtraining is referring to a form called Sympathetic Overtraining. There is however, another form of overtraining, Parasympathetic. These two forms of overtraining present differently. Endurance athletes most likely have Parasympathetic, and so if they are trying to determine whether or not they are in a state of overtraining most of the information that they are receiving isn't even relevant to their condition, and hence may lead them astray.
Athletes involved in ANAEROBIC activities should watch for signs of Sympathetic stimulation. The Sympathetic Nervous System controls the "Fight or Flight" response and other stress responses. This means that their morning heart rates may be elevated.
Athletes involved in AEROBIC sport, should be cautious of Parasympathetic stimulation. The Parasympathetic Nervous System controls "Rest and Digest" functions. This means that the athletes morning heart rate may NOT be elevated at all, and they may be still in danger of being overtrained. I mention HR specifically, because this one aspect of overtraining is the indicator mentioned in nearly all easily accessible information regarding overtraining, but for endurance athletes, their morning heart rate would most likely present in the opposite, not elevated!
The meat and potatos here is that if you are feeling stale, overworked, etc. TAKE TIME OFF. The body cannot differentiate between training stressors and work stressors.
Taking a daily mood and motivation inventory is the best tool research has found to determine overtraining. You may even begin to add to your stress level by stressing about NOT training! TAKE TIME OFF.
The following link is the best article i have read on overtraining to date. It is research backed, and fully encompasses all details of overtraining syndrome. For the most part, it is NOT anecdotal, and when it is hypothesizing, the author states so. Check it out!
Train Hard, Recover Well!