...during early January in New York, such marathons aren’t unusual for dancers. The reason — or the culprit, some might say — is the phenomenon known as APAP. While that acronym technically stands for the Association of Performing Arts Presenters, a 60-year-old national organization that convenes each January in New York, it has become shorthand for the ever-expanding constellation of festivals and showcases surrounding the association’s conference.The conference, which begins on Friday, brings together hundreds of performing arts producers and curators from around the country and world for a frenzied five days of networking, talent-scouting and shopping for live performance. The proceedings include industry-specific meetings and discussions, along with ample opportunities to roam the Expo Hall at the Hilton in Midtown, a maze of booths where artists, agents and managers can pitch their work or their clients’ to presenters.