The phrase "first marathon" refers to the very first time a person runs in a long-distance race that is 26.2 miles (42.2 kilometers) long. Full definition
Tranter, 58, has been looking over the Park District's shoulder for more than two decades, starting as an organizer of Chicago's first marathon in 1977 during the 13 - year reign of Supt. Edmund Kelly. (articles.chicagotribune.com)
I have a marathon in a few weeks (first marathon as a «no meat athlete») and my last marathon (my first) I totally hit the wall at mile 22. (runyourbq.com)