No running back in the NFL has been more explosive this season than Tennessee’s Chris Johnson. He leads the NFL in rushing with 1,396 yards and is on pace for a 2,000 yard season and a shot at Eric Dickerson’s NFL record for rushing yards in a season. A former Marine is a one of the keys to Johnson’s success. Titans fullback Ahmard Hall, who served four years in the Corps, is quietly earning a reputation as one of the top blocking backs in the NFL, as ESPN’s Len Pasquarelli writes. Hall, like most fullbacks, rarely carries the ball and…