When I wrote my OP, I had "practice doesn't make perfect; perfect practice makes perfect," running through my head. The strength training analogy would be, as soon as you can't lift the weight with perfect form, reduce the weight or stop. Otherwise, you're grooving sloppy, dangerous form into your nervous system. (On the meditation side, maybe that would be grooving unproductive mental habits into one's mind.)
Similarly, my motivation for practicing the way I am is to strongly condition myself, right from the beginning, "to meditate when I'm meditating," or "stay on protocol." In MCTB, Dan says, "It is important to know that really getting
into a sense of the breath as a continuous entity for 10 seconds will do
you more good than being generally with the breath on and off for an
hour" (139). That really, really hit home for me and was probably my main motivation for all of this: to really nail those 10 seconds (or one minute, or one hour) over and over again. When I think of those hours upon hours upon hours of "meditating" in lala land, well, let's just say that now I know better, and I'd prefer change my ways.