I agree with others and want to second BrainLe's comment about gear. I found that I walk a natural stroll at 2mph. My fastest sustainable walk would be 3mph. I just know that my experience.

I've never thru-hiked like Bearpaw or BrainLe or others (but I've actually read both of their journals - good stuff) so certainly listen to what they have to say. I know that when I switched from boots to trail shoes, my stride was affected. I felt lighter, faster and more agile. It was fun!

When the going gets tough I inhale for two steps, and exhale two steps; when it really gets going I inhale for 3 steps and exhale two steps. It really helps me get my mind off the hill too.
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I always forget and make it more complicated than it needs to be...it's just walking.