I dont think its particularly good for you. But its not terrible either.
well theres 3500 calories in a lb of fat. I guess you burn about 1800 calories just sitting doing nothing. Add to that your 800 burned. is 2600 take the 1500 calories eaten away from that is 1100 calories burned a day.
so 20lbs * 3500 calories = 70,000 calories you need to burn. divide that by 1100 you burn a day is 63 days.