Most treadmills run on DC motors with magnets in them, so they could generate all the power you like if you pushed the treadmill and had the right controller.
I used to have a cheap one with no motor at all. It was slightly inclined. You just pushed backwards to get it started and it used your own momentum/gravity to turn.
And treadmills now probably consume way more power now than they could actually generate.
Most treadmills run on DC motors with magnets in them, so they could generate all the power you like if you pushed the treadmill and had the right controller.
I used to have a cheap one with no motor at all. It was slightly inclined. You just pushed backwards to get it started and it used your own momentum/gravity to turn.