RaycastHit2D.distance

Description

The distance from the ray origin to the impact point.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Vector2 direction;

    void Update()
    {
        //Cast a ray in the direction specified in the inspector.
        RaycastHit2D hit = Physics2D.Raycast(this.gameObject.transform.position, direction);

        //If something was hit.
        if (hit.collider != null)
        {
            //If the object hit is less than or equal to 6 units away from this object.
            if (hit.distance <= 6.0f)
            {
                Debug.Log("Enemy In Range!");
            }
        }
    }
}