Dedicated to Mara's travel and hiking adventure journals as well as her words of wisdom and suggested resources for hikers and travelers.
Foot size changes
The pounding long-distance hikers put on their feet in a few months is similar to the wear and tear most people put on their feet over many years. As you age, the connective tissue in your feet loosens and your feet tend to get wider, longer, and flatter. Depending on your physiology, this can happen in a relatively short period of time on the trail. Your weight and foot length can also be indicators of what to expect. Heavier people and longer footed people can expect greater change.
Anecdotal stories from any individual is unfortunately likely to bear little resemblance to what anyone else is likely to experience.
A friend on mine who had done multiple thruhikes without a change in foot size did a long hike in 2005 and had his feet change significantly while on the trail.
Myself? I was diagnosed with what doctors and physical therapists call "loose muscles" when I was in high school. My feet have changed over two full sizes, primarily during the course of my Appalachian Trail thruhike. They tightened up a bit after my AT hike but spread again, and further, while on a large section hike of the Pacific Crest Trail and haven't gone back at all since.
If you're wearing trail shoes on the trail, the shoes themselves are often flexible enough to conform to your feet and stretch quite a bit while you wear them. Once the shoes wear out, hikers are often surprised to find the same size shoe that they are throwing away that was perfectly comfortable no longer fits when they get a new pair. Once again, it probably depends on what type of shoes or boots you are wearing. If you are wearing heavy rugged boots, they are less likely to conform to your feet as your feet grow and, if your feet spread significantly, you may need to replace them before the boot wears out.
To plan replacement shoes on the trail, consider going into a town with an outfitter after just a few hundred miles. Do this even if your first pair of shoes is still working for you. This will give you some idea how your feet have already changed (or not) and will help take the guesswork out of shoe selection when you get further up the trail. If your feet haven't changed a bit by then, perhaps they are less likely to start changing dramatically on the trail. If your feet have already changed quite a bit by then, they may either continue to change, or be done changing already.
Even if you don't need them yet, this could be a good time to buy shoes to mail to yourself up the trail if there aren't any outfitters up ahead.
As for how many pairs you'll need on the trail, people who wear trail shoes need more than people who wear heavy boots. Those with lighter packs can usually go with lighter shoes. Unlike a boot, trail shoes need very little break in time, if at all. Heavier people, longer footed people, and people who tend to "kick rocks" may need more frequent replacements than others.
You may also want to read my write-up about choosing "Hiking boots vs. Trail shoes."
Last updated, June 28, 2013.
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