Every now and then I like to write a little piece about just how terrifying it can be to write something big and impressive, and offer some advice on just what you will encounter on that journey. Of course, every adventure has its own unique set of challenges, so let's not worry about those. Let's worry about the ones that I see on chat pages and Q&A sites about writing all the time, because more than a few people have these kinds of setbacks. And you know what? They're not that troublesome once you realize everyone hits these obstacles.
I have a bunch of scenes I want to write, but how do I fill in the spaces in between them? This is sometimes referred to as the Tentpole problem - the tentpoles are the key events that hold up the story, but the story can't just weigh down the space in between the poles. For these moments, I refer to the periods during wartime between the major battles. This time is best utilized by implementing the three R's of warfare - recover, reorganize, reload. After a major scene, you can have the characters recognize what happened, consider whether it changed their pursuit, then prepare to move forward. This allows the space between the tentpoles to still borrow from the excitement of the main scenes, but also carry the reader along without weighing down the story. And I will mention this again - you can always rewrite it.
How many words should my story be? I always like to say it should be no more than one story long, but sometimes people need a little more guidance. A novel can be as short as 50-60,000 words; anything shorter is usually considered a novella. I don't aim for a word count in my stories or chapters, I look to tell the story I want to express through a series of scenes that each have their own message. If it makes you more comfortable, set a word count for each piece. However, the most important part should be pacing above all else. Forcing exciting chapters to end prematurely or extending simple bridge chapters will destroy the reader's experience. As you write more and more, you will get a feel for the proper length of your piece. (Pro tip: If your manuscript is above 200,000 words, you probably need to trim it down.)
I don't know how to end this. Yes, this happens, and it usually happens because the story you set out to write changed along the way. At this point you need to think about your story in two sentences - the conflict you main character is facing and what they should experience at the conclusion. Once you write down those two sentences, your objective is to make sure that last sentence is satisfied. Then, at that point, see if the first sentence is still part of your story. Welcome to the rewriting phase.
There are dozens of common pitfalls and obstacles on the writer's journey. The only advice I can give that will cover them all is that you don't let any of them stop you from writing down something. Anything. Anything you write, you can rewrite, just never lose the momentum. Keep on writing, one word at a time, to the very end.