Android : How to correctly use a Workerthread?

I've been writing android apps for some months now, and I'm at the point where I'm building an actual needed app.

As I want that to work nice and fast, I made a Workerthread to do all kinds of tasks in the background while the UI can...build up and work and stuff.

It's based on the Android Studio Drawer app blueprint.

In Main.onCreate I got my operator=new Operator(), which extends Thread.

Now, when loading a new Fragment, it sometimes calls MainActivity.operator.someMethod() (I made operator static so I can use it from anywhere), and after some time I realized, the only tasks actually running in background are those in the operators run() method and an Asynctask my login Fragment runs. Everything else the UI waits for to complete and therefore gets executed by the UI thread.

So I thought: no problem! My operator gets a handler which is built in run(), and I change those tasks:

public void run() {
    Looper.prepare();   //Android crashed and said I had to call this
    OpHandler = new Handler();
    LoadLoginData();
    [...Load up some Arrays with hardcoded stuff and compute for later use...]
}

public void LoadLoginData() {
    OpHandler.post(LoadLoginDataRunnable);
}
private Runnable LoadLoginDataRunnable = new Runnable() {
    @Override
    public void run() {
        if(sharedPreferences==null)
            sharedPreferences= PreferenceManager.getDefaultSharedPreferences(context);
        sessionID=sharedPreferences.getString("sessionID", null);
        if(sessionID!=null) {
            postenID = sharedPreferences.getString("postenID", PID_STANDARD);
            postenName = sharedPreferences.getString("postenName", PID_STANDARD);
            context.QuickToast(sessionID, postenName, postenID);
        }
    }
};

context is my MainActivity, I gave the operator a reference so I could send Toasts for Debugging.

But now, the Runnables seem to not run or complete, any Log.e or Log.d stuff doesn't arrive in the console.

After some googeling and stackoverflowing, everyone is just always explaining what the difference is between Handlers, Asynctask, and Threads. And the multitask examples always only show something like new Thread(new Runnable{run(task1)}).start times 3 with different tasks.

And so became my big question:

How to correctly, over a longer time (~lifecycle of the MainActivity), with different tasks, use a background thread?