It works better as a negative predictor than a positive predictor. If you're planning to run 3:05 in the marathon, and you can't average 3:05/800m, then you probably won't be able to meet your goal.
But it doesn't indicate anything about your overall marathon preparedness. If you were training for a 5K, you would be able to run 3:05/800m easily, but if your long run was only 12 miles, it's very likely you'd hit the wall in the marathon and not be able to run 3:05. If your overall marathon training, mileage, long runs, etc. have been going well, then you can feel much more confident about your 3:05.