There is somewhat less genetic differences than you might think, same with the east African runners. Scientists don't find any obvious advantages the east-Africans have, ever so slightly lighter legs and smaller feet but nothing obvious like high red blood cell count, increased heart volume, lung size etc. (and those latter attributes are adapted during training anyway). It is mostly just down to condition. as you say, a child that has to walk miles ot get water, 5 miles to school, help out ona hunt going 30-50 miles in a day. Doing that every day from the moment you can walk means that covering a marathon distance is quite trivial. The Tarahumara Indians aren't genetically unique, siply long distance running is part of their culture so without even specific training they can win 100 mile ultra-marathons wearing sandals and a skirt.
Genetically we are all evolved to be able to run massive distances efficiently, climb mountains etc. Our bodies are really exceptionally well engineering for long distance running, which is why we can outrun most animals over a long enough distance. That is in apart how our ancestors hunted, simply chasing an animal until it collapsed or overheated allowing us to get into spear range. Just in modern society we sit around all day and do exercise for fun rather than survival, and that makes a big difference in how far we can push ourselves.
