Complex sentence: The Army Ten-Miler is the second largest ten-mile race in the USA, after the Philadelphia Broad Street Run and held every October in Washington, DC and sponsored by the U.S. Army Military District of Washington.
Split into simpler sentences: The Army Ten-Miler is the second largest (after the Philadelphia Broad Street Run) ten-mile race in the United States. It is held every October in Washington, DC, sponsored by the U.S. Army Military District of Washington.