Very true. You don't "overtrain" in only one training session. Not to mention the fact that the term is "overused". Finding how hard you need to push can very often include finding where your limits are. Pushing the envelope is what those who pass up "average" take part in regularly. Unless you are fatigued or your training is mismatched with your food intake and rest, worrying about overtraining could also have the effect of holding you back.
In other words, unless your aproach to training is completely clueless, I would worry more about how I ate that day.