Running out the clock
In American football, each quarter of a game is measured with a 15-minute game clock, or 12-minute clock in many high school football codes and the German Football League. A team in possession of the lead and the ball will attempt to use up as much of the game clock as possible in order to bring the game to an end more quickly, thus denying the opposition another chance on offense.
Comparison of American football and rugby league
Generally, American football games last much longer than 80-minute rugby league matches. Because the field is reset after each tackle in American football, it is much slower-paced than the more hectic rugby league, in which play stops for only as long as it takes the tackled player to get back to his feet and return the ball to play. Another major difference is that only the player with possession of the ball may be interfered with in rugby league; defending players interfering with any other attacking player (and vice versa) will incur a penalty. Passing in the two sports also differs: while backward passing is common to both sports, in American football players are allowed to throw the ball forward in certain situations whereas in rugby league forward passing is always illegal. rugby league footballers have to kick the ball or run it forward to advance it downfield. To score a try or touchdown in rugby league, the player has to push the ball directly onto the ground once in the try zone, whereas in American football the player has to simply cross the line. American football also requires its players to use a large amount of protective equipment, including helmets, gloves and padding around the body, whereas protective clothing is much more minimal in rugby league, usually amounting to light padding (if any) and soft head gear (hard helmets being illegal). 
The rugby pitch and American football field are both similar in size with a pitch being 100 meters while a football field is 100 yards.