You might have been fortunate enough to have an agent request pages for your book. You may have taken a lot of time to make them better first. For example, if you're writing crime fiction (a mystery) you may say
1. My book is too long today
2. I don’t want my mystery to be obvious.
3. My plot is intricate, so I’m wary of severing the links throughout.
Those are all related. Your book is probably running as long as it does so it will contain everything to keep the plot bolted together. The complexity of the plot makes a mystery deeper, for example. If it’s longer for any other reason, it becomes a bit easier to cut. If it was a piece of seasoned beef, it might be overseasoned with characterization or scenes that run long.
That effect of “goes on too long” is a matter of taste and talent. Even when you’re writing well, you don’t get as many extra pages as you think. You get more pages, but you have to keep readers turning those pages.
If you want to be double sure that your plot is durable, you will need a second check. That’s a set of outside eyes. I'm talking development editing, not copy editing or line editing.
Letting your story loose into the world is the solution to these problems.
"I want a book that holds together and keeps the reader wondering what’s going to happen." That’s noble. It can be a road sign toward complexity, of course, depending on how many subjects are in play. Your book should have a primary story mission, and that mission had better fulfill the protagonist's desire.
Just because a book’s structure has come together over years of work, like it does for most of us, doesn’t mean it can’t get streamlined. I think here about the rivets in the planes that Howard Hughes built for competition. Always streamlining. He set records, his accomplishments you can see in The Aviator.
I once edited a book from 140,000 words to 75,000. The author went too deep in many passages and her protagonist was inside internal monologue at great length.
I edited my novel Viral Times down from 144,000 to 98,000 words. To do this, I discovered Scrivener and used it to identify what was in the book and what could go. It helped that I’d already worked 30 years as a copyeditor. Cutting isn’t easy, but it feels good after you face it down. At some point every creator has to have some compassion for readers who, frankly, would like to get to the next book, either in the series or from another author. Savoring a big novel is a delicious thing, of course.
It also helped that I performed in and watched many hours of theatre — where the dramatic arc includes nothing but scenes, and they each must have good work to do to serve the narrative.
Lots of first books founder on the shoals of trying to become perfect. If yours is going to get an agreement to represent from an agent, or a contract with a publisher, it’s going to change anyway if you’re lucky. Another pilot in the wheelhouse is a good thing, eventually. Is today the day you’ll throw open the wheelhouse door and set a deadline to submit?
Deadlines are essential for me. I just don’t finish without them. I write them down in a fine diary (Passion Planner!) and enjoy checking them off.
In 1984 I had a job in a typesetting house. At Graphics Express we promised delivery by the end of the day. Those were deadlines we often missed, and my job was to call the customers to ask for extensions. I did it every afternoon, my least favorite part of that work day. As if turns out, it’s the least favorite part of my writing day, too.
Send your book to your outside reader thick, even if it's 110K. You'll streamline it. Michael Connolly, a superior crime writer, has a first book at 143,000 words. He’s better than most of us, earned a Pulitzer, and then sold that book after writing two others that were never published. Length isn’t a killer if the plot is serving the characters. We care more about people than events.