At this point, if you press the down key quickly and let go, the character will complete the animation of one stride and then stop, which is what I wanted to do.
The character will move whenever "vertMove" contains a value, and will stop moving if it goes to zero.
Inside the "WalkD" movie clip I've set vertMove=2 at the beginning of the animation, and then made it zero at the end of the stride. So in theory, he should keep moving at a speed of 2 until the stride animation completes.
This isn't true though. If you tap the down key once he sometimes just moves a little bit and stop while the animation completes. Adding dynamic text to watch the vertMove variable shows that it returns to zero before the animation is over, even though I ONLY mess with the value inside of that animation...