Triathlon Update: Knowing When to Say When

One of the ideas our triathlon coaches have been repeating throughout our training is that its better to be undertrained than even a bit overtrained. I fully understand their point. If you're overtrained, you're either going to end up injured or burned out. In both of those cases you won't perform well come race day. That being said, I have a hard time making myself stop training.

For tonight's practice, we had a swim at La Jolla Shores. Going into the practice, I had already swam ~5000 yards this week. After swimming ~1800 yards, I wanted to go back out and swim a bit more. Our coach Dave told me that since we've got a practice tri in the morning, I should just call it good for the evening and head home. I did end up following his advice. However, it took evey bit of will power to do that. After thinking it over, I've identified several reasons for this:
1) I felt great after swimming the 1800 yards. Going back to the swim practices we used to do in high school, a practice doesn't feel complete until your body is begging for you to stop.
2) On a purely quantitative level, 1800 yards just doesn't feel like a full workout. Granted most of my swim workouts these days are between 2000 and 3000 yards. But again my mental standard for swim workouts is based of the 6,000-8,000 yd workouts we used to do back in high school.
3) Even though it goes against the mantra of training smarter not harder, my gut instinct is that it takes more training volume to get better. Yes, I know that is wrong. But this is what my gut tells me and it takes a lot of work to ignore it.

Because of all of these, the feeling that came over me to night when I left (with my body feeling great no less) was one of guilt. Part of me felt like a quitter. But hopefully as I learn to take a more targeted and scientific approach to training, I will find it easier to ignore this voice.

Have any of you had common experiences?