Epidurals are done in an attempt to reduce inflamation thereby alleviating pain (if the pain is caused by inflamation) as to how it will help determine where your pain is, IF the pain stops or lessens, obviously this is the area causing you pain. It is primarily used as a diagnostic tool to help zero in on your problem. But of course if it brings you relief it is also considered to be therapeutic.
Most likely he WILL do different areas "bit by bit" he'll not go in and inject you everywhere at once
please keep us posted on how your doing & Good Luck to you