A hymn or a piece of organ music often contains a particular passage that just seems to “settle in” at only one tempo. It is often a passage that contains much rhythmic activity, or a dense concentration of small note values. Mark this passage, and then hear it in your mind at “that certain tempo” before you begin.
This is especially important to remember if the beginning of the hymn or piece consists mainly of larger note values. Beginning too fast could ruin the rest of the performance.