Novels generally have a three-part structure: beginnings, middles, and ends.
Beginnings connect the reader to the main character, present the story world, establish tone, introduce the opposition, and compel readers to move on to the middle.
Endings wrap up all the strands of the story, give the outcome of the final conflict, and leave a sense of satisfaction and resonance.
Middles develop the confrontation between the main character and the antagonist, deepen character relationships, keep us caring about the main character, and set up the final conflict.
Middles keep the main character and the antagonist in conflict. If one or the other can simply walk away, there is no reason for the reader (or writer) to muddle through the middle. Duty can be the adhesive keeping them in conflict (a detective needs to solve a case). Moral obligation can be the adhesive (a character exacts revenge or a mother fights to save her child). Physical location can be the adhesive (a blizzard makes it impossible for the characters to leave a place).
Middles have a rhythm of action, reaction, more action, and how these beats are controlled determines the pace of the novel. Lots of action, little reaction gives a breathless pace. Little action, lots of reaction slows the pace.
Middles should have a sense of suspense, a sense of death hanging over the main character (can be physical, psychological, professional, or moral), and a sense of increasing risks and rising stakes.
Here are a few questions to keep in mind as you muddle through the middle of your novel:
What adhesive do you use to keep your characters from being able to walk away?
How do you vary the rhythm of action and reaction to create the pace of your novel?
Does your novel have suspense, some question to be resolved, something that will keep readers paging through the middle?
Do you have a sense of death hanging over your main character?
How do you keep increasing the risks for your character?
How do you keep raising what is at stake for your character?