At 285 lbs should I be embarrassed that I can only sustain running at about 4.5 mph on the treadmill? How fast should I be running after training for 6 months and losing 20 lbs?
Not at all. Do what you can, start where you are.
That's a hard question to answer, weight isn't the only factor. That said, an often cited figure is that you should lose about 2 seconds per mile for every pound of extra weight you drop. I wouldn't focus on speed though. Keep most if not all of your work easy. (zone 2). If you're main goal is to get faster and weight is secondary to you, I would recommend limiting your faster sessions to only once per week. That's all you need to do to get faster assuming you are doing easier days the rest of the week. The cardio system doesn't need constant hammering. Just doses here and there.
The most success I've had losing weight over the years is when I combine a long endurance low intensity exercise regime with a slightly hypo-caloric diet. The idea here is not for performance, but for weight loss. People push HIIT training, but while HIIT or high intensity training does burn more calories, it leaves you with a ravaging hunger that's difficult to tame. There is a saying in the running world...you can't outrun a bad diet. Diet is about 75% of the equation. But don't get too aggressive with it or you'll get sick or injured or burned out. Your body has a way of adapting to what you give it too. If you give it too little calories it can slow your metabolism as a defense.