I recently ran and walked to Mordor -- 1,779 miles in 50 weeks. It was a great experience. More recently I've been experimenting with taking a completely different mental approach to running, and then comparing my subjective experience that to the objective data I can collect from my Fitbit Charge HR.
My new approach is to try to run as if "nothing is happening." In other words, I want to think to myself "I will run" in exactly the same casual way as I would think "I will make myself a cup of tea." It isn't an unusual thing, it doesn't involve effort, it's no big deal, it's just a pleasant thing to do.
A few days ago I went for a short 2 mile run using this thought process. My goal was to stay completely relaxed, not ever feel out of breath, not ever feel like I was "trying" much, just feel like I was being "pulled along" by the world, as I maintained high alertness without stress.
I completed this run, which felt very easy. Later I looked at the data and I was shocked by what I saw. I had spent 15 minutes at above 150bpm heart rate. I know from some of my other workouts at the gym that normally even 1-2 minutes at 150bpm makes me feel like I am pushing myself too hard and I should back off immediately. But this felt like nothing.
I have known for a long time that one's mental approach can make a big difference in athletic performance, but I have rarely experienced anything quite as dramatic as this. It is really interesting, and I will keep exploring this way of running to see what else I can discover.
If you enjoy walking or running, I'd encourage you to try this. It is a bit hard to explain the key part of it . . . it is running as if the running part is not even the main thing you are doing. The running part is just a mechanism for exploring and experiencing the world in a very happy way. Try this and see what happens.