The key to hiking many miles day after day is to figure out a steady pace that fits you.
One of the best things you can do is to get a heart monitor and figure out how to maintain a steady heart rate and pace on a trail in the mountains. It is confusing when you start out with the variance of uphill, level and downhill in the mountains. A light or heavy backpack will change your heart rate and exertion level. A heart monitor measure your actual level of exertion whether you are training or out on a big backpacking adventure.
If you measure it, you can change it!
Learn how to vary your pace on uphill, flat and downhill to keep a steady heart rate. What you want to avoid is a high heart rate on the uphill that causes you to go anaerobic and to start burning a high percentage of glucose (sugar) for your energy. An anaerobic pace and level of exertion is not sustainable if your goal is to hike 20-25 miles for multiple days. The object is to stay in your aerobic, fat burning, zone as much as possible. You have a large fat supply, but only a limited glucose supply. You can only eat and digest food so fast. And it is harder to digest while you are hiking down the trail.
With the help of a heart monitor you will begin to understand your pace, breathing rate and develop a habit of a sustainable pace under different conditions of the trail and your pack weight. Eventually it will become a habit and you won’t need a heart monitor to tell you whether your pace is too fast or too slow.
Hiking poles can also help you maintain a steady pace and rhythm. Check out the hiking pole category.