NavMesh flee. Ai flee from player.

Right sorry for the delay @Marks981 (work got in the way), this isn't perfect although you may not worry about the drawback which I'll explain after posting the code. Oh and there's a lot of comments in there so the actual code is a lot smaller:

 using UnityEngine;
 using System.Collections;
 
 public class NavMeshRun : MonoBehaviour {
 
     private Transform player;
     private NavMeshAgent myNMagent;
     private float nextTurnTime;
     private Transform startTransform;
 
     public float multiplyBy;
 
 
     // Use this for initialization
     void Start () {
 
         player = GameObject.FindGameObjectWithTag ("Player").transform;
         myNMagent = this.GetComponent<NavMeshAgent> ();
 
         RunFrom ();
     }
     
     // Update is called once per frame
     void Update () {
 
         // used for testing - can be ignored
         if(Time.time > nextTurnTime)
             RunFrom ();
     
     }
 
     public void RunFrom()
     {
 
         // store the starting transform
         startTransform = transform;
         
         //temporarily point the object to look away from the player
         transform.rotation = Quaternion.LookRotation(transform.position - player.position);
 
         //Then we'll get the position on that rotation that's multiplyBy down the path (you could set a Random.range
         // for this if you want variable results) and store it in a new Vector3 called runTo
         Vector3 runTo = transform.position + transform.forward * multiplyBy;
         //Debug.Log("runTo = " + runTo);
         
         //So now we've got a Vector3 to run to and we can transfer that to a location on the NavMesh with samplePosition.
         
         NavMeshHit hit;    // stores the output in a variable called hit
 
         // 5 is the distance to check, assumes you use default for the NavMesh Layer name
         NavMesh.SamplePosition(runTo, out hit, 5, 1 << NavMesh.GetNavMeshLayerFromName("Default")); 
         //Debug.Log("hit = " + hit + " hit.position = " + hit.position);
 
         // just used for testing - safe to ignore
         nextTurnTime = Time.time + 5;
 
         // reset the transform back to our start transform
         transform.position = startTransform.position;
         transform.rotation = startTransform.rotation;
 
         // And get it to head towards the found NavMesh position
         myNMagent.SetDestination(hit.position);
     }
 }

The downside is this, I couldn't find a way to get the forward information to be away from the player without pointing the object in that direction first. Now this might not bother you, having the enemy/creature suddenly face away from the player might look fine in your game, in which case not a problem.

I tried recording the start transform and once we get the hit point on the navmesh setting the position and rotation back to the startTransform, but it does appear to glitch slightly when finding the position.

It might just be that I'm using an empty scene with cubes so it's more obvious on screen.

Give it a shot anyway.