What a great exercise! Very quickly I found that I had to concentrate on the subject, to the point of seeming to do blind contouring drawing. Super fun.
I practiced a gesture drawing of horses in the neighbouring field, and realized that after they moved on, and some of horses looked liked elongated sausages, that this would have been the perfect application for measuring a head, and then using that measurement as a tool to refine my gesture drawings. Not having done that, all I know is that my ‘sausage horses’ aren’t in proportion, but I don’t have the info to correct them. Great tip for using the tools at hand!