Personal Heatmap question

I was hoping to make a heatmap of all my walks, most of which are private activities or have the maps hidden, so the 3rd party heatmap tools don't pick them up (and I have little desire to go back through hundreds of activities to change their privacy settings just for this.)

I was disappointed to see that I can't filter the personal heatmap to only show walks. Is there really no way to do this?