Creating a HUD that indicates how close the player is to an NPC

Hi,

I am very new to unity and learning from online sources. I am creating a maze based game as part of a PhD on human threat processing.

I am trying to create a HUD that shows the user how close an NPC is to the players position. I want to use a slider that fills up and changes from green to red as the NPC gets closer to the player. I have managed to create a basic HUD that works in stages (code below) but want this to be a smoother and not staged. I have tried to use Lerp but have struggled to make it work. The below script is attached to a rigidbodyFPS. Can anybody offer any advice?

Cheers,

Brad

 public class PlayerScript : MonoBehaviour
 {
     private GameObject triggeringNPC;
     private bool triggering;
     public GameObject npcText;
     public Slider DDSlider;
     public Transform NPC;
     float DDDistance;
     int CircaDist = 1;
     int PostDist = 2;
     int PreDist = 4;
     
     void Start()
     {
         DDSlider.value = 0;
    
     }
 
     void Update()
     {
         if (Vector3.Distance(transform.position, NPC.position) <= CircaDist)
         { DDSlider.value = 100;
             GameObject fill = DDSlider.transform.GetChild(1).GetChild(0).gameObject;
             Image fillImage = fill.GetComponent<Image>();
             Color newColour = new Color(1f, 0f, 0f);
             fillImage.color = newColour;
         }
 
         else if (Vector3.Distance(transform.position, NPC.position) <= PostDist)
         { DDSlider.value = 50;
             GameObject fill = DDSlider.transform.GetChild(1).GetChild(0).gameObject;
             Image fillImage = fill.GetComponent<Image>();
             Color newColour = new Color(1f, 1f, 0f);
             fillImage.color = newColour;
         }
 
         else if (Vector3.Distance(transform.position, NPC.position) <= PreDist)
         { DDSlider.value = 25;
             GameObject fill = DDSlider.transform.GetChild(1).GetChild(0).gameObject;
             Image fillImage = fill.GetComponent<Image>();
             Color newColour = new Color(0f, 1f, 0f);
             fillImage.color = newColour;
         }
 
         else
         { DDSlider.value = 0;
             GameObject fill = DDSlider.transform.GetChild(1).GetChild(0).gameObject;
             Image fillImage = fill.GetComponent<Image>();
             Color newColour = new Color(0f, 0f, 0f);
             fillImage.color = newColour;