using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Transform target;
    public float maxHorizontalDistance = 10;

    void Update()
    {
    if(target == null) return;

        var distance = target.position - transform.position;
        distance.y = 0;
        var magnitude = distance.magnitude;
        if(magnitude > maxHorizontalDistance)
        {
           transform.position += distance.normalized * (magnitude - maxHorizontalDistance);
        }

        transform.LookAt(target);
    }
}