How do i make my character not "instantly" get to and from max speed?

Hello I have been self scripting the code to my game, "the carpet Rouge" and I have ran into a tiny little detail... Whenever I press the up and down keys my character instantly goes to max speed (20), and when I let go of the key he does the same thing. I want him to slowly build up to the max speed, and then slowly slow down into an idle state! Do not mind the (keycode.space) that is just a booster function for my game

here is the script, HELP!?

 #pragma strict
 
 
 var movespeed : float = 5;
 var rotateSpeed : float = 5;
 var jumpHeigth : float = 10;
 var smoothDamp
 function Start () 
 {
 
 }
 
 function Update () 
 {
 if (Input.GetKey(KeyCode.UpArrow))
     transform.Translate(Vector3.forward * movespeed * Time.deltaTime);
     
 if (Input.GetKey(KeyCode.DownArrow))
     transform.Translate(-Vector3.forward * movespeed * Time.deltaTime);
 
 if (Input.GetKey(KeyCode.RightArrow))
     transform.Rotate(Vector3.up, rotateSpeed * Time.deltaTime);
 
 if (Input.GetKey(KeyCode.LeftArrow))
     transform.Rotate(Vector3.up, -rotateSpeed * Time.deltaTime);
     
 if (Input.GetKeyDown(KeyCode.Space))
     movespeed = movespeed + 15;
 
 if (Input.GetKeyUp(KeyCode.Space))
     movespeed = movespeed - 15;
 
 }