Maybe switch exercises two and one? The first exercise could be "just sit down, don't try to think or not think...just take notice of what arises in your mind"

meditators who are almost universally astonished to discover how much narration is going on in their head when all the distractions are removed--and they often aren't told explicitly about subvocalization.

I too, experience an endless stream of thought. It's just that it is largely nonverbal thought and therefore not described by the term sub-vocalization. But it's still thought, and it is similarly problematic in its capacity for distraction (if not more so), and would probably benefit from meditation.