The voltage that your treadmill needs to run is lost on long extension cords. Not only that but light duty cords can get very hot and cause fires or other damage.
The very best situation is to plug your treadmill’s power cord directly into a dedicated 20 amp outlet. Then keep the treadmill unplugged when its not in use to prevent any electrical surges from damaging your treadmill.
If you do not wish to be bothered by unplugging your treadmill after each use, then I strongly suggest that you use a dedicated surge protector (see image below). This plugs directly into the outlet with no additional cord, and nothing else can be plugged into it, like with power strips.
By the way, you can get these Belkin SurgeCubes on Amazon.com for $8.84. If you are an Amazon Prime Member you can get it with free shipping.
If your treadmill cannot be moved close enough to plug it into the wall, then the ideal situation is to have your electrician wire a dedicated 20 amp outlet that is within reach.
If you must use an extension cord, then use the shortest cord length possible, and use at least a 12 gauge power cord, like the yellow one in the picture above.
If you have someone that is handy then you can make the exact length cord that you need by purchasing a length of 12 gauge, 3-wire cord, and the male and female plugs, like I did with the yellow one in the picture at the top.