Safety first!! Learn to quickly shift the transmission to neutral if you have "unattended acceleration". Practice doing that while your driving. It won't hurt the transmission and it will prepare you for an emergency. Since Toyota is fixing similar problems, try the dealer, but since your car is over 10 years old, I myself would first spray some WD-40 all around the accelerator cable linkage at the intake manifold (top of engine). Temporarily remove the driver's floor mat to make sure it's not causing the problem. You may need a new accelerator pedal cable.
This problem could also be due to a sticking throttle plate as suggested in the first answer. So I agree that the throttle plate and bore should be cleaned and check for free movement as in the first answer.
This problem could also be due to a sticking throttle plate as suggested in the first answer. So I agree that the throttle plate and bore should be cleaned and check for free movement as in the first answer.