How to start a timer on keypress

Hello I have been making a sprinting script today and all is good except for the fact that I could hold lshift all day and it would still carry on running. So all I ask is could someone tell me how to do a timer on keypress this is what I have got so far. Yes I am using the default character controller.

var timer : float = 10.0;
private var motor : CharacterMotor;

// Use this for initialization
function Awake () {
	motor = GetComponent(CharacterMotor);
}

// Update is called once per frame
function Update () {
	// Get the input vector from kayboard or analog stick
	var directionVector = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
	var  sprint : int = 7;
	var reg_speed: int = 4;
	timer -= Time.deltaTime;
	
	if (directionVector != Vector3.zero) {
		// Get the length of the directon vector and then normalize it
		// Dividing by the length is cheaper than normalizing when we already have the length anyway
		var directionLength = directionVector.magnitude;
		directionVector = directionVector / directionLength;
		
		// Make sure the length is no bigger than 1
		directionLength = Mathf.Min(1, directionLength);
		
		// Make the input vector more sensitive towards the extremes and less sensitive in the middle
		// This makes it easier to control slow speeds when using analog sticks
		directionLength = directionLength * directionLength;
		
		// Multiply the normalized direction vector by the modified length
		directionVector = directionVector * directionLength;
	}
	
	// Apply the direction to the CharacterMotor
	motor.inputMoveDirection = transform.rotation * directionVector;
	motor.inputJump = Input.GetButton("Jump");
	//Speeding Up
	if (Input.GetKeyDown(KeyCode.LeftShift)){
	motor.movement.maxForwardSpeed = sprint;
	}
	if (Input.GetKeyUp(KeyCode.LeftShift)){
	motor.movement.maxForwardSpeed = reg_speed;
	}
}
// Require a character controller to be attached to the same game object
@script RequireComponent (CharacterMotor)
@script AddComponentMenu ("Character/FPS Input Controller")

to make a timer on a key press do this

//this is equall to 10 seconds
var timer = 10;

if(GetKeyDown.KeyCode(" whatever key you want"){
timer -= 1 * Time.deltatime;}

if (timer <= 0)
do whatever;