So when you're starting the middle, think of how the protagonist can be challenged. That is, they exist to make things happen in the plot, but their own conflicts and issues shouldn't be part of the story. But in most protagonist-centered popular fiction, tracking the secondary characters' lives and loves is going to waste time and confuse the reader. Watch out for long passages in a secondary character's viewpoint which dwell on his problems and not on the protagonist. But that doesn't mean that effective pacing depends on shoving a lot of events into the story.
Selection is key.
But it's death to good fiction. Fiction is about cause and effect, and there's no cause and effect when the central elements of your plot happen by coincidence. It's often hard, however, to identify coincidence in your own story, so be ruthless. Look at the chain of events. To fix coincidence without losing the event, make it happen because of character decision and action, and watch your characters grow into strength and purpose.