Some people naturally write short books - this can be anywhere from 55k words up to 75k. These people are good at getting to the point and at diving right into conflict. Then there are those of us who revel in complication. Our books are long, usually 100k. Possibly more. Possibly much, much more. We wouldn't know simple if it started chewing on our faces. I feel it's important to acknowledge that neither is superior to the other. Stories are still the result and there is no perfect length for a book. So before we dive into how not to bore yourself to tears whilst writing a longer book, let's acknowledge that not everyone is cut out to write long books. Just like I am not cut out to write short. No. That's wrong. I can write short. I have done. What I cannot do is write simple. I'm allergic to straightforward plots without dozens of other threads woven through. The one time I forced myself to do so the plot was - well - weak. So don't feel badly if you start a long book and it just doesn't work. It may not be your strength. Don't volunteer to be the fish who tries to climb a tree. With that painful metaphor etched in your brain, let's talk long books.
Long books need a lot of plot. They need extra conflict. They need bigger stakes and bigger problems to be solved. You probably won't find many stories about saving the world or all life on earth that run about 55k words long. It just usually takes a little longer to get to there. Longer books are where you bring in secondary story lines involving secondary and tertiary characters - so long as it all contrasts or reinforces the main story. In longer books, complications breed complications, raising tensions and obstacles for characters to overcome. This also means that your characters have a lot to conquer in themselves. Whatever their flaws or weaknesses that keep them from solving all the problems right now, they need to be either deep seated enough or the character obstinate enough to need extra time (and extra pain) to bring about real change in the character. It's a lot to juggle and it's what you'll need to keep yourself tuned into the rise and fall of conflict across the long expanse of words you have to write. So how do you keep from getting bored? You don't. Sorry to break it to you but when you write longer books, you get bored. It's just part of the process. You've been in the story for so long, mucking around in the workings, solving problems, working out the bumps and stops, there's simply no human way to not get sick to death of it. You will. So your only hope is to plan for it and to push through it. Unless. Unless you can take a pause and look for a twist even you didn't see coming until you go to this 'wow, I hate this story' spot. Sometimes it works and you'll plow on with renewed energy and a mental note to rewrite your synopsis. The rest of the time, you just have to embrace the pain of 'story doesn't care how you feel, hush up and put the words in'. The good news is that your boredom will rarely last past the ramp to the 3/4 crisis. The other good news is that just because you're bored, it doesn't necessarily follow that readers will be. You're bored because familiarity breeds contempt. You're too close. Knowing that won't dispel boredom, but it might be enough assurance to get a few thousand more words out of you. The only other advice I can offer is to remember to turn into conflict. Your characters might not like pain, but you need to love it for them. Think of a long book as your villain origin story - learn to enjoy torturing your characters (and thereby your readers) for fun and profit. That makes the long slog a bit more entertaining.