My wife purchased a used Bowflex Treadclimber 3000 2 weeks ago. The last two times she has used it, it has popped the GFCI after an hour of continuous use. It is the only appliance plugged in to that circuit, and is plugged in via a 50-ft. grounded indoor/outdoor extension cord. The circuit is original with the house; 11 years old. Is the circuit going bad with age? Do I have a problem with the machine? Do I really have a problem at all?

Thanks.

First loose the 50 foot extension cord. One for safety purposes. the other is that GFI devices can trip due to the length of cords.

Per the instruction manual:
http://download.dfxi.com/supportdocs/AM/Bowflex/BFX_TC135_AM_2008.pdf

The unit draws 12 amps, and it allows that " a short cord is permissible".

50 foot is not short, and should not be used for a 12 amp load for extended periods.

Since the unit is used, it can be have a problem. Could be the GFI device.

I suspect the issue is the cord.

Long cords have high capacitance that can trip a GFI for no apparent reason.