The M8 chip in the iPhone is what records your step data, and it always does it. The Health app is just a portal to view data. If you want to see your step data, add the Step dashboard in the Health app. If you don't want to see it, don't add the Step dashboard.
I'd be interested to know ultimately how much memory this is going to use. If you look at the actual data, it's taking data captures every minute or two and saving them in a log that goes on forever. And mine has only been tracking data for a few days. Health app is showing it only takes up 2.2mb so far, but I wonder what this is going to look like in a year if it saves it all. I can't seem to manage storage in the health app through settings.
It's based on the region you have set up in phone settings, so US shows miles. Kind of silly -- I feel like there should be a setting to change it to imperial or metric, but it doesn't look like one exists (yet).