How to make my character limited run?

Hello I customised the fps input controller so I could run. Here is the way I modified it but I want to know how do I limit how long I can run!

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;

 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)