Carterusm - Good question but theres no great science involved.
The body constantly gives great feedback about what it wants/needs - most runners seem to assume there is 'something wrong' when the feedback from the body is along the lines of 'okay, you're doing great but can you just give me a bit of a breather to let me catch up'.
For marathon training, it is important to train on tired legs but that would normally mean paces being maybe 30-60 secs/mile down on what you might expect to run when fresh. Maybe a bit of a struggle at times but no problem with completing runs.
When getting into overtraining territory the body starts getting more assertive along the lines of 'look mister, when I said I needed to catch up I wasnt joking, so either back off or I'll ultimately have to make you do so'.
This can mean legs feeling very, very heavy and/or a strong demotivation to train. Dont fight it, just keep runs steady or have a rest day or two until normality returns.
But importantly, it doesnt mean that anything is wrong, it is the normal process of the training stress/training effect cycle.
The worst approach is being fixated on weekly mileage totals or having to religiously 'follow a schedule' above all else and ignoring the obvious signs given by the body.
Many runners will know instinctively where the line is and will build in cutback weeks in anticipation of needing to.
Another mistake would be to never get anywhere near the overtraining line by being ultra cautious, doing so means depriving the body of the training it needs to give its best performance.
Edited: 04/02/2013 at 13:14