Hi Jason.
The 120 volts is the required power (housepower) for the plug.
The treadmill motor is dc and will handle up to 90 volts dc and will turn up to 5,000 rpms.
So in theroy if you spin it 5,000 (five thousand) rpms you will get back, the specs on the motor.
A safe blade rpm is about 500 (five hundred). some treadmill motors will give you (about 20 volts) at (about 1,100 rpm's). mine does. Gearing would depend on you wind area.
Turn your motor with a drill, or something find 20 volts. and note the rpm's.
Search the board for treadmill motors. You will have lots to read.
Note: I'm finding treadmill motors don't last very long in to out doors.