How do I make my movement script move the character UNTIL I let go of the key?

Hello, I am quite new to Unity in general and was wondering how I can this movement script move the object until I let go of the key. So far, if I hold A or W, it moves the object 1 step at the desired speed, but only once. I have to let go of the key and press it again to repeat the process.

   using UnityEngine;
     using System.Collections;
     
     public class controlScript : MonoBehaviour {
         //Variable declaritions
         static float speed = 5;
         static float step = 1;
         private Vector3 pos;
         
         void Start () {
            pos = transform.position;
         }
         //Actual Control
         void Update () {
             if (Input.GetKeyDown(KeyCode.D))
             {
                 pos.x += step;
                 
             }
             if (Input.GetKeyDown(KeyCode.A))
             {
                 pos.x -= step;
             }
     
             transform.position = Vector3.MoveTowards(transform.position, pos, speed * Time.deltaTime);
     
     
     
     
         
         }
     }

Thank you in advance