How is it possible for pain I have had for so many years to go so quickly?

Speed of recovery in treatment depends, to a large degree, on how complex the causal factors are, in any condition.

For instance Fred fell and knocked his back out of line ten years ago, it wasn’t painful so he didn't get it treated at the time and since parts of his back have become rigid to accommodate his postural imbalance, and this rigid muscle is now causing him pain. Although a long time has passed since Fred's initial accident, and compensation patterns have built up in his body, there is no additional complexities to his condition as he is otherwise very healthy, generally stress free and isn’t under great physical stress. So the only two things to do are release the compensation patterns and realign the spine from the original injury. As there are no contemporary causes for his condition when the fascia and muscle tissues are released in treatment his body and can return to a good balance and his spine can re align in no time at all.