PHILADELPHIA—An enduring image of American film is Rocky Balboa triumphantly mounting the steps of the Philadelphia Museum of Art after failing his first attempt in the 1976 Oscar-winning "Rocky."

By "Rocky II," when the underdog boxer repeats the feat surrounded by cheering children, he hardly seems winded.

But astute locals have long noticed something amiss about that run in the 1979 sequel. The montage shows Rocky, played by Sylvester Stallone, starting from his South Philly home, then quickly cuts to him running along train tracks—roughly 7 miles to the northeast. Then, he is back in South Philly's open-air Italian Market, and before you know it he is miles away along the Schuylkill River.

The run through distal parts of the city seems almost impossible, even for someone as tough as Rocky.

Enter the ultra-running movement to show it is possible. Nearly four decades after the first Rocky movie, a group of runners set out Saturday to re-create Rocky's training run—all 31 miles of it, the equivalent of 50 kilometers.