Changing a NavMesh Destination

I have a nav mesh on my enemy, I would like the enemy to choose a point, walk to that destination, wait 5 seconds and then choose a new destination. This works once, but when the enemy is due to choose a new location, it chooses a new destination every frame resulting in him moving all around the map very quickly. What changes should I make to get the enemy to choose a new location only once?

     void Update () 
     {
          StartCoroutine(PathRemaining());
     }

     void WalkPosition()
     {
         Vector3 randomDirection = Random.insideUnitSphere * 15.0f;
 
         randomDirection += transform.position;
         NavMeshHit hit;
         NavMesh.SamplePosition(randomDirection, out hit, 15.0f, 1);
         Vector3 finalPosition = hit.position;
 
         agent.SetDestination(finalPosition);
 
         Debug.Log(finalPosition);
     }

     IEnumerator PathRemaining()
     {
         if (!agent.pathPending)
         {
             if (agent.remainingDistance <= agent.stoppingDistance)
             {
                 if (!agent.hasPath || agent.velocity.sqrMagnitude == 0f)
                 {
                     animate.SetBool("Idle", true);
                     //Debug.Log("Done");
 
                     yield return new WaitForSeconds(5.0f);
                     WalkPosition();
                 }
             }
             else
             {
                 //Debug.Log("Walking");
                 animate.SetBool("Idle", false);
             }
         }
     }