Getting For Loops to work in a Coroutine

Hello,

I have created a function that has two for loops inside for altering a terrain. The function itself works fine but it makes the game hang while it is processing. To fix this I have changed the function to be a co-routine with a yield in the for loop so it returns to function update after each pass.

After several Debug messages placed about the code I have determined that the co-routine is being called by StartCoroutine() but the for loop isn't. Anyone know why that would be?

Here is a stripped down version of the code.

     void Update ()
     {
 
         if (generate)
             
             StartCoroutine(GenerateLandmass ());
             
             Debug.Log("Setting Seed");
             
             generate = false;
         }
         if (newTerrainInfo)
         {
             Terrain.activeTerrain.terrainData.SetHeights(0,0,heightMap);
             newTerrainInfo = false;
         }
     }
     
 
 
 
     IEnumerator GenerateLandmass ()
     {
         //this shows in the console
         Debug.Log("Started world gen co-routine");
         
         for (int y = 0; y < worldSize; y++)
         {
                         //this doesn't show
             Debug.Log("row");
             for (int x = 0; x < worldSize; x++)
             {
                 DoStuff();
                 yield return 0;
             }
         }
         newTerrainInfo = true;
         
         
     }