If you were in marching band, there's a good possibility that you had more thorough training in marching than what's given in basic training, especially if you went to competitions. Marching makes up like half the activity of marching band (it's in the name). Marching is only one of a plethora of things that are taught during the few months of basic training, and once you're out of basic, you may never have to march again.
I also think your expectations on how rhythmically-inclined the average person (or soldier) is might be on the high side based on your experience in an activity with a bunch of highly rhythmically-inclined people.