“As someone who has lost touch with reality, I like to hold a firm grasp on it now,” Mary Cain says while we walk through a palm-tree spotted campus in California. She’s telling me why she insisted she write her own memoir, This Is Not About Running, without ceding the narrative to a ghostwriter, as happens with many athletes. “My story is so complicated … there are so many bad actors that I think it forces the reader to embrace nuance, and I don’t think you see that very often.” At 29, Mary Cain is a decade removed from her experience as the United States’ highest hope for a middle-distance track star since Mary Decker smashed women’s world records up and down the stat sheet in the 1970s and 80s. Cain set four different national high school records as a teen, and as a 17-year old made the world championships in the 1500m, finishing 10th in a field of pros.…