Yes, all of the above! I have multiple treadmills, and the most expensive one (and all or most of the expensive ones) have a reed or hall sensor to adjust the speed just like I am proposing here. But with their own boards and logic of course.
The reason I know it is off is because I have filmed it and estimated the speed of the belt by using frame rate and travel distance of a marker. The expensive treadmill is dead on. One of our newer cheap ones is s bit off, but the one I am using as a "daily walker" so to speak is off by much more.
Granted, the main reason for it being off by so much has to be that it has overheated a few times and the motor magnets are probably not what they once were. I might change the motor some day too. I've already started to modify this treadmill with cooling and everything, so I just thought this would be a cool project.
I walk while I work on the computer, and I have made a tracking web app too. Just to track my sessions and send them to Garmin. But with an Arduino in place I could even control the treadmill directly from my web app.
None of this is really needed just to be active while working, but it's a really fun challenge and I want to see if I can make it happen!