Help With Basic AI, Keep Specific Distance From Player

I have just begun an independent study on Unity at my University, so I am very new to this. I have successfully created an AI which will more towards the player, collide with the player, and do damage to the player. I am now trying to modify this script in order to create a new enemy type which will move up to the player, but then maintain a constant distance from the front of the player and fire projectiles.

Here is what I have tried:

using UnityEngine; using System.Collections;

 using UnityEngine;
 using System.Collections;
 
 public class yellowCubeAI : MonoBehaviour {
     
     public Transform target;
     public int moveSpeed;
     public int rotationSpeed;
     private float distance;
     
     private Transform myTransform;
     
     
     
     void Awake(){
         //caches transform
         myTransform = transform;
     }
 
     // Use this for initialization
     void Start () {
         GameObject go = GameObject.FindGameObjectWithTag("Player");
         
         //set target to player
         target = go.transform;
         
         //calculate distance
         distance = Vector3.Distance(target.position, transform.position);
     }
     
     // Update is called once per frame
     void Update () {
         Debug.DrawLine(target.position, myTransform.position);
         
         //look at player
         myTransform.rotation = Quaternion.Slerp(myTransform.rotation, Quaternion.LookRotation(target.position - myTransform.position), rotationSpeed * Time.deltaTime);
         
         //move to target
         if(distance>=20){
             myTransform.position +=myTransform.forward * moveSpeed * Time.deltaTime;
         }
         else if(distance<=19){
             myTransform.position +=myTransform.forward * (-1) * moveSpeed * Time.deltaTime;
         }
     }
     
     
 }
 

As of now this AI behaves just like my other, and simply runs into the player. Any help is appreciated!