Table long shots need to be hit perfectly straight, otherwise by the time the cue-ball reaches the opposite side of the table you can be as much as a centimeter off. That could mean the difference between making your target ball and totally missing the pocket.
Test out the straightness of your stoke. To help you with the aiming, use the head string and the point on on table to line up the shot. It is also a good idea to use a striped ball instead of the cue-ball.
If you are within 1-3 inches of your cue, smile. Unless you are using brand new cloth stretched tight it is very difficult to have the cue-ball come perfectly back consistently. The rail bumper could even through the shot off.
Positioning your target ball with the stripe riding vertical and stroking straight through it you will notice if the shot isn’t straight because the line will wave instead of run smoothly.
Think about performing this shot along the width of the table in the beginning, and work your way to the full length of the table when you are confident enough.
The stoke should be hit with medium speed to make sure that inconsistencies in the felt don’t push it off course. Since the distance back to your cue is great even a crumb could turn into a few inches.