I'd like to share some insights I've gained over the years for those who have expressed an interest in hiking longer distances. Much of this self found knowledge came from training for ultramarathons, but can be applied to just about any training situation where one wants to get more out of the "engine."
Many training books/methods talk about setting up your "foundation", be it running miles or weight training or biking etc. The theory behind this is you have to build from a base, increasingly stressing your body as it adapts to the training. One thing that is not discussed, however, (at least I haven't found any references yet) is the concept of "body memory." Recently, I found myself a bit overweight for my build, out of shape from a bad muscle tear, and toting a newfound set of "moobies." In January I started training again. Understand that I did not go hog wild in the gym, I hit it about 3 days a week, sometimes 4. Were I starting from scratch, meaning no prior conditioning, there is no way I could have come up to speed as quickly as I did. Within a month I was able to run 12 miles, and I attribute this to the "body memory" concept. Once you attain a certain level, or take your body to a certain extreme, the bod "remembers" this and is able to go there again much easier than the first time, even with no substantial conditioning. After I ran my first marathon I was a total wreck, never having experienced this sort of strain on the body before. Subsequent races became much easier, and felt familiar, with no increase in training. One year I showed up late for a 24 hour race and quit early, logging 67 miles on the track. My wife had to bathe me as I sat on a stool in the shower, unable to stand myself. The next year, with essentially the same training schedule, I put in 104 miles on the same track. No recovery problems at all. Granted, your body adapts over time and becomes stronger, but I truly believe that once you've "been there", going back (even after a significant amount of time has elapsed with no training) is much easier because of this "body memory." What does this mean for hikers? If you've only done say 17-20 miles in a day, you have to plan a shock session. Start early, go late, but go for 25-30 miles on a day hike. Expect a very hard day, as you have never been there before. You may have "rabbit leg", as my daughter calls it, where the legs start thumping from hours of going downhill. You will be tired, and you'll wonder how you are ever going to make it back to the car. But once you've put in a day like this, the next time will be easier both from the physical standpoint but more importantly from the mental aspect and the experience of having been there.
Ah, the mental aspect, something I consider more important than the physical. You have to adjust your way of thinking about what is achievable and getting away from the "standards." Like, "oh, I could never do 30 miles per day, the most I've ever done is 20." Until you do 30. Or maybe the most you've ever done is 10, in which case maybe you want to be able to do 15 miles or more. All the same concept. Mental toughness is paramount for those who want to go longer. Days of leisurely hiking, starting from camp at 10 AM and arriving at your next camp at 3 PM is a favorite way to enjoy the trail, but if you want to cover more distance you have to readjust some things. Get up early, get on the trail by 6, and you can cover 5-7 miles before you stop for breakfast. Plan on arriving at your campsite late, enjoying your meals on the trail, and you can cover another 5-7 miles over a typical hiking day. There is going to be a certain amount of suffering involved when you push your body to go beyond your comfort zone, but if you accept this as part of the equation then it just becomes familiar territory, not to be feared, and you will find yourself going longer than you once thought possible. In effect, you have established a new norm for yourself. Through recent experiments I have found that the mental aspect stays embedded, able to be dredged up when needed. And physically, you can go back to that familiar territory even though you are older, fatter, and less fit than before. As a final example, one year I was running Western States, and around mile 70 my legs were floppy and I could barely make it down the trail. I convinced myself that there was no way to recover, that once I crossed the river at around 75 miles, I would drop out of the race as my crewman was going to meet me there. So we met up and I sauntered up to the next aid station, short of 80 miles, and told the volunteer there that I was dropping out. She said, "are you sure you want to quit, you have 2 minutes before the cutoff time." Shamed thusly, I said " OK" and continued on, picking up a stray pacer along the way. And then I had a miraculous recovery, literally bounding down the trail with the pacer barely able to keep up, feeling better than I had even at the beginning of the race. How could this be? Just before I arrived at the last cutoff aid station, around 85 miles, I heard the cutoff horn. I had missed the last checkpoint by a minute, and was not allowed to continue. I was a DNF that year not because I was physically unable to do it, but because my mental weakness had cost me that extra minute. Armed with this knowledge and experience, the next year I had no such lapses, and even through violent puking in the canyons and occasional floppy legs, mentally I was full focus and this made all the difference for a successful completion.
Want to go longer on your hikes? You gotta train both your body and your mind, the two are inseparable. Shock your system by doing something extreme (for you). Learn how your mind can overcome physical discomfort, that mental toughness is your most important tool. Enjoy your wilderness time in any manner you deem appropriate, including leisurely hikes, but I would also encourage you to experience the other side, where you cover long distances, thousands of vertical, and experience the trail on the trail, not just in a campsite. It doesn't matter your physical makeup, as you establish your own "norm", but know that you can go far beyond what you think is possible.