I wish it was that simple. I can visually see hogs running at 3 different speeds and the distinction between the three can't always be grouped in MPH. The low speed run is a trot where they are just moving away and don't seem to understand what it going on. There is a mid speed run where they appear to be fleeing, but not actually reacting like they are about to die. They are just moving away from danger, quickly. Then there is the balls-to-the-wall 30 mph run where they feel directly threatened like they are being chased.
When you shoot at them, the don't always go from zero to flat out full speed. That is part of the problem. On top of that, they don't always run at a constant speed once they start running.
Then there is the problem of vertical changes. Hogs often have a lot of vertical change when they run fast. Depending on the timing of the shot and how much change is being produced, your head level shot could impact at belly level.
It is going to take practice. It would probably be beneficial if you tried to learn from a shorter distance.