Moving GameObject a specific distance in the Z direction and back again - regardless of rotation

DLively - Thank you for the help! It seems I can get my object to move in the correct right direction every time using this logic. However, the problem I face is determining when I have traveled the preset distance of 1f in Z. Here's the code I am using now to move it and determine if I've reached the distance I need to. It doesn't seem to work correctly as sometimes it will overshoot the distance or sometimes not go far enough depending on the rotation in Y of the game object. Am I calculating the distance traveled wrong once it starts moving?

 startingPoint = new Vector3(transform.position.x, transform.position.y, transform.position.z);
 zDistance = 1f;
 comeback = false
 
 void Update ()
 {
 if(!comeback)
 {
 Debug.Log("I'm moving forward");
 lastPos = transform.position;
 distance = lastPos - startingPoint;
 zDistanceTraveled = Mathf.Abs(distance.z);
 Debug.Log("Total Distance I Must Go:" + zDistance);
 Debug.Log("Distance I've Traveled:" + zDistanceTraveled);
                 
  if (zDistanceTraveled >= zDistance)
 {
 comeback = true;
 }
 else
 transform.position += transform.forward * Time.deltaTime * speed;
 }
 }