But the middle doesn't have to be feared, either! Why? Because you have room, space and time.
Unlike the beginning, which needs to hook the reader and set up the plot and subplots, and unlike the end, which needs to tie up all storylines and leave a feeling of satisfication, you have a wide-open avenue throughout your middle. Your subplots have been established; your characters are introduced and your climax is your fixed point to drive toward. This is the time to explore every possibility that your fictional world can allow. You don't have to worry about consequences just yet (leave that to your ending). Pull the lids off all those broiling ideas and uncork the possibilities. You have room to do it!
These possibilites apply to everything. Introduce new settings to help refresh your story; with the space you have to spin out your story through your middle, you can take the chance to more fully develop and realize the locales in your world. You can introduce new characters or deepen the dynamics between current ones; there's space to add quirks, kinks or wrinkles to relationships.
Your beginning is what enables your middle; your ending needs to tie up everything you've developed in your middle. Those are big responsibilities. But during your middle? Take your time. You've got the room and space to do it.
What are the hardest/best aspects about middles for you?