I have a solid intro, along with a solid ending, but how do I connect the two and keep it interesting without making it seem like I'm just doing busywork and saving all of my creative energy for the ending?
It depends on what sort of story and genre it is that you have, but one approach I like to bear in mind as I try to think about middles is that the person who your main character is at the start of the novel should not be able to overcome the challenge they face at the climax of the novel. So how does their character change and grow over the course of the novel? What knowledge and experience will they need to acquire to overcome that challenge? Then it becomes a matter of deciding the events and milestones that need to happen on the road to changing that character.