The root canal procedure should not be as painful as imagined. Do not worry. 
As you must know by reading this article, root canals can be painful.
Most of the time when you need a root canal you are already in pain. Th reason is because there is an infection in yours tooth and the root canal procedure is needed to remove both the pulp and nerve from the tooth to take out the infection.
The root canal itself will hurt about as much as getting a cavity filled.
The Novocaine needles can be somewhat painful when the needle pin prick and bee sting feeling happens, but that should be the end of the pain while in the dentists chair. The drilling of the tooth will not be painful but can be traumatizing as that sound of the drill spinning is terrible.
You should not be feeling any pain until well after your appointment and the doctor will have prescribed some pain pills to solve this problem.