* Please note that this post is based on my own experience and observations and is not based on scientific research or data.
The trick is to really give your mind a break on these walks. Don't focus on anything in particular. Simply relax your mind and enjoy your surroundings. It is best not to listen to music to maximize the effect of this and restore your mental faculties. Music can be enjoyable, but it is a distraction and doesn't help in the process as much. You want to be free of all sensory input with the exception of the surrounding environment.
It is also best to be surrounded by nature, with many trees and plants. The oxygen emitting from them is a big help and promotes a purifying of the mind.
You will find that many insights and thoughts will go through your mind. Just observe them and allow them to flow on. This is the process of emptying your mind and processing all that you have previously taken in through reading and studying. Your brain is making sense of all this data and also recharging and recalibrating itself.
After a half hour walk, I find that I am able to settle back into my reading and studying for another lengthy period of time. This wouldn't be possible without these restorative walks.
It should be noted that reading and studying for long periods of time also takes practice. Focus and concentration is a mental muscle that must be developed over time through ever-lengthening sessions of practice. It is a very rewarding power to harness. The feeling of satisfaction and accomplishment one receives after a day of many hours of reading and studying is powerful. However, to really boost these periods of immersion, you need to take breaks to restore your mind. Walking in nature is a great way to accomplish this.