He played in the National Football League (NFL) for 4 seasons from 2006 to 2009, with his longest tenue as a player with the Chicago Bears.

After graduation from high school in 2002, he enrolled at the University of Oklahoma and played running back for the Sooners.

He played 4 seasons as a fullback in the NFL, with the Chicago from 2006 to 2007, the Tampa Bay Buccaneers in 2008 and the Cincinnati Bengals from 2008 to 2009.

On August 16, 2008, Runnels signed with the Tampa Bay Buccaneers after the team released offensive tackle Luke Petitgout.

His release was profiled on the season premiere of the HBO series Hard Knocks: Training Camp with the Cincinnati Bengals.

On March 17, 2022, it was announced that Runnels was hired as the Co-Offensive coordinator/Running backs coach of the Houston Gamblers of the United States Football League.