The Try/Fail Cycle
Writers often complain about writing the middle of a book, but to me, it's quite simple. The middle of the book is almost always an expanded try/fail cycle in which the main character(s) throw themselves at the goal at hand and fail over and over again in many different ways. It's your job as the writer to think up all the different ways in which they can fail because obviously they can't keep failing the same way over and over again (that would be boring).
A character's goal from the beginning of the book is to become an expert swordsman so he can lead his father's troops into battle. Great. Your job as a writer is to make this as difficult as possible. Why? Not because you're mean, but because readers want to read about difficult things. They want to experience triumph through your characters, and the greater the difficulty of the process, the keener the feeling of success will be at the end. Also, just to be realistic, if your character were to succeed at first try, you would not have a novel on your hands. It would take ten pages.
Becoming an expert swordsman is a big goal. There are many problems to deal with. A character is going to have to build up muscles (think Mr. Miyagi). That will take time. It will be painful. There will be moments when the character wants to give up. Or moments when the goal seems impossible. People may laugh at the main character. This is all the stuff of the middle of your book. It's not boring and it's not hard to write. It's just not about success.
Once muscles have been built up, the character is going to have to spend time practicing with a real sword, against real opponents. This won't be fun, at least not very often. Get the biggest, baddest opponents for your main character that you can. They're not going to take it easy on them, either. There will be defeats in spades. And more wanting to give up. More sore muscles, as well as cuts, bruises, and other injuries.
After the physical problems of learning to use a sword will come other problems. Or these can come before, too. What if someone doesn't want the main character to succeed and tries to forbid lessons? Or tries to sabotage a lesson so that it ends fatally? What if there is something else that stands in the way—a food shortage, a premature attack by the bad guys, a call home to deal with a problem there? Someone dies unexpectedly, or one of the main characters really does give up and go home, leaving the others to scramble to fill in the gap. Betrayal. Loss. Grief. This is all the stuff of novels. Novels are full of problems, not so much solutions.
Try not to think of your novel as a straight shot from beginning to end. It's a very complicated journey with a lot of unexpected problems that your main characters have to deal with. Showing them become strong enough to deal with problems that get progressively worse is what writing a book is. Sometimes you can even introduce the biggest problem at the beginning and let it look smaller, but as the main characters get smarter, they begin to realize the truth of the task ahead of them in a way they hadn't before.
Readers want to journey with these characters. They want to see them struggle because it gives readers hope that they can deal with the problems in their own lives. They also want to feel like reading your book is worthwhile. They want to experience the fear of failure and then the glory of success through your characters. It makes us all feel like our lives are more valuable when we see that difficulties can be overcome in time, with a great deal of effort, and with friends at our sides. A book is meant to *feel* real, even if it isn't real in any way. It should be emotionally real, no matter how fantastical the setting.
One of the biggest problems I see among beginning writers is the misunderstanding of what the middle part of a book is. What ends up happening a lot is that I feel like there are five or six first chapters setting up small problems, and then a long set up of the climax, which ends quickly, and there's a lot of denouement. In fact, this is paced completely wrong. The beginning sequence of a book should be fairly short, and so should the climax and denouement. It's the middle part of the book that takes so long. If, as you write it, it feels like it's taking far too long, ignore this feeling.
Hopefully you will have readers who can tell you if what you're doing is working or not, but the middle part shouldn't feel boring or like it's taking too long. It should feel like all this is absolutely necessary, or if there are parts that are taking away from the main goal, like the reader is desperate to get back to the rest. This is all an illusion, of course. You as the author are the illusionist here, planning out each next step so that the reader will follow you to the end, believing that all of this is as it must be because your characters could do nothing else in the world you've presented as theirs.
Obviously, no one expects you to get this try/fail cycle one hundred percent right the first time. In revision, your job is to figure out what parts go by too quickly, without enough complication, and what parts need to be taken out or made more exciting. You're also going to be in a constant hermeneutic circle with your end and beginning. You will need to tweak or change them as your middle changes. And you will need to change the middle as you realize what the best ending is, and where you must begin to get there. Sometimes you have to look at all the different stories you can tell and choose just one of them.