Movement of character by holding the up arrow instead of repeatedly pressing it.

hi, i am following the design3 tutorial on a skateboarding game but they have it so you press the button down to go a bit faster every time you press the button. i would like to change it so i press the button down and hold it so the character will reach its maximum speed then continue to move forward for a period of time like in any tony hawks game. im very new to coding so any help would be appreciated. Here is the code i am using

 var rotateSpeed = 90;
 var pushImpulse = 3.5;
 
 
 private var character : CharacterController;
 private var trans : Transform;
 private var speed = 0.0;
 function Start ()
 {    
     character = GetComponent(CharacterController);
     trans = transform;
 }
 
 function Pushing()
 {
     speed += pushImpulse;
 }
 
 
 function Update ()
 {    
     var horizontal = Input.GetAxis("Horizontal");
     trans.Rotate(0,rotateSpeed * horizontal * Time.deltaTime,0);
     
     if (Input.GetKeyDown(KeyCode.UpArrow))
         Pushing();
     
     character.Move(trans.forward * speed * Time.deltaTime);
 }