Treadmill goes full speed on start up.

I own a well used Reebok Edge REOM 11301 series treadmill. As soon as I switch it on the treadmill immediately goes top speed. I cannot turn the speed down and have to pull the emergency key out to stop the machine. I have looked everywhere for help and think maybe the problem is the speed sensor? I cannot locate the speed sensor to see if it has broken. I would really appreciate some guidance.

Many thanks.