BOSTON — Guy Morse, who led the Boston Marathon into the prize money era and helped preserve it as the world’s most prestigious 26.2-mile race, is retiring from the Boston Athletic Association after 28 years.

Morse was race director of the Boston Marathon, the B.A.A.’s signature event, for 15 years before spending 10 years as the organization’s executive director. He has been the senior director of external affairs since 2010.

Morse helped usher the marathon from its days as an amateur race with about 5,000 runners into an annual spectacle with 25,000 participants and more than $800,000 in prize money.

Morse also presided over the creation of a year-round schedule of B.A.A. races, which now includes a half-marathon, 5K and 10K.