How can I make my object move faster while holding a keybutton

Ok. Im doing a 2D game movement but I want my character to move alot faster then usual while holding the key. Here is my code. Im a begginer, so please >.< can you explain and tell me where to add the code ^w^ Thank you!~

public var MovementSpeed : float = 5;
public var JumpSpeed : float= 10;
public var MoveDirection = Vector3.zero;
public var Gravity : float= 50;
public var grounded : boolean =false;

function Update () {

	if( grounded){
		MoveDirection = new Vector3(Input.GetAxis("Horizontal"),0,0);
		MoveDirection= transform.TransformDirection(MoveDirection);
		MoveDirection*= MovementSpeed;
	
	}

	if (grounded){
	
		if(Input.GetKey(KeyCode.Space)){
		
			MoveDirection.y = JumpSpeed;
	
	   	}
	
	}
		MoveDirection.y -= Gravity*Time.deltaTime;
		var Controller = GetComponent(CharacterController);
		var Flags = Controller.Move(MoveDirection*Time.deltaTime);
		grounded=(Flags & CollisionFlags.CollidedBelow)!= 0; 

}

Hi what do you mean by the code ^w^ .An to speed up jus add this part to your code
create a ibput button called sprint

var runspeed:float = 8;/////any value
    function Update(){
    //////////your movement code
    if(grounded && Input.GetButton("Sprint")){
    MoveDirection = new Vector3(Input.GetAxis("Horizontal"),0,0);
           MoveDirection= transform.TransformDirection(MoveDirection);
           MoveDirection*=runspeed;
    
    }
    
    }