Function to make a new TerrainData and assign it to a selected Terrain GameObject.

createterraindata.cs

//Two editor functions you can put wherever you like

public void CreateTerrainData()
{
    //get the selected terrain gameobject
    //create a terrain asset in the folder selected in the project view
    //name the terrain data the same as the gameobject
    //assign the terraindata to the terrain and the terraincollider

    var t = Selection.activeGameObject.GetComponent<Terrain>();

    if (t == null)
    {
        Debug.LogError("No terrain component on selected object. Aborting."); 
        return;
    }

    var tcol = t.gameObject.GetComponent<TerrainCollider>();
    if (tcol == null)
    {
        Debug.LogError("No terrain collider on selected object. Aborting."); 
        return;
    }

    var tgoName = t.gameObject.name;
    TerrainData terrainData = new TerrainData ();
    const int size = 513;
    terrainData.heightmapResolution = size;
    terrainData.size = new Vector3 (2000, 600, 2000);
    terrainData.heightmapResolution = 512;
    terrainData.baseMapResolution = 1024;
    terrainData.SetDetailResolution(1024, terrainData.detailResolutionPerPatch);
    var selectedProjectPath = GetSelectedPathOrFallback();
    AssetDatabase.CreateAsset(terrainData, selectedProjectPath + tgoName + "_terrainData.asset");
    t.terrainData = terrainData;
    tcol.terrainData = terrainData;
}

public string GetSelectedPathOrFallback()
{
    string path = "Assets";

    foreach (UnityEngine.Object obj in Selection.GetFiltered(typeof(UnityEngine.Object), SelectionMode.Assets))
    {
        path = AssetDatabase.GetAssetPath(obj);
        if (!string.IsNullOrEmpty(path) && File.Exists(path))
        {
            path = Path.GetDirectoryName(path);
            break;
        }
    }
    return path + "/";
}