Will a throttle cable stretch over time?

I've got a chart here that shows stress limits, stretch, etc.. for a lot of cable types. Braided wire rope, spiral wounds, all different. Anyway... Your foot effort wouldn't stretch it very much at all.
Unless you bought the car new you can't be sure of stock everything. Replacement cable or change in throttle pedal assembly are my guesses. Your condition is somewhat common where a slant 6 cable is used on a V8.