During the early 1920s, Marshall grew interested in promoting Adirondack recreation. In 1922, he became one of the charter members of the Adirondack Mountain Club (ADK), an organization devoted to the building and maintenance of trails and the teaching of hiking in the park. In 1922, he prepared a 38-page guidebook, entitled "The High Peaks of the Adirondacks." Based on his pioneering experiences on the peaks, the guide recommends that "it's a great thing these days to leave civilization for a while and return to nature." Marshall provided a brief description of each peak and arranged them in order of "niceness of view and all around pleasure in view and climb."