As I'm working on my newest book, I'm reminded again of the importance of pace in a story. In my stories there are quite a few heavy action scenes, and young readers like those, but we can't maintain a fast pace without wearing down a reader after a while.
Every good action scene needs to follow with a substantial rest period, a time for reflection and setting up the tension that leads to the next action scene. Without the buildup of tension, the action seems frenetic and without purpose. It's simply tiring.
Also, I think the reader needs some time to catch his breath with the characters. The reader enjoys a few pages to contemplate the consequences of the action with the characters and how they must plan for the future. As each action scene is threaded together by skillful rest periods, the reader feels the motivations behind the action and is emotionally involved throughout.
So, always remember to give readers a break, one that helps them emote with the hero as he cleans his sword, savor the aroma of the meal the hero is cooking over an open fire as he chats with his comrades, and feel the energy as the hero mounts his horse for the next adventure.