Running C#

Hello i really want my player to be able to run, but dont want to use the basic movementscript that you get from unity packaged. So heres my movement script, Thanks for the help (:

using UnityEngine;
using System.Collections;

public class Movement : MonoBehaviour {
	
	public float movementSpeed = 5.0f;
	public float mouseSensy = 5.0f;
	public float jumpSpeed = 20.0f;
	
	float verticalRotation = 0;
	public float upDownRange = 60.0f;
	
	float verticalVelocity = 0;
	
	CharacterController characterController;
	
	// Use this for initialization
	void Start () 
	{
		Cursor.visible = false;
	}
	
	// Update is called once per frame
	void Update () 
	{
		CharacterController characterController = GetComponent<CharacterController> ();
		
		//Rotation
		
		float rotLeftRight = Input.GetAxis("Mouse X") * mouseSensy;
		transform.Rotate (0, rotLeftRight, 0);
		
		verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensy;
		verticalRotation = Mathf.Clamp (verticalRotation, -upDownRange, upDownRange);
		Camera.main.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);
		
		//Movment
		
		float forwardSpeed = Input.GetAxis ("Vertical") * movementSpeed;
		float sideSpeed = Input.GetAxis ("Horizontal") * movementSpeed;
		
		verticalVelocity += Physics.gravity.y * Time.deltaTime;
		
		if(characterController.isGrounded && Input.GetButton ("Jump"))
		{
			verticalVelocity = jumpSpeed;
		}
		
		Vector3 speed = new Vector3 (sideSpeed, verticalVelocity, forwardSpeed);
		
		speed = transform.rotation * speed;
		
		characterController.Move (speed * Time.deltaTime);
	}
}

So, make some button called “run”, and use runSpeed or speed+runSpeedBonus or something when Input.GetButton(“run”) is true or something? Not really seeing the problem here.