If you had to choose, which would you pick? I mean a legitimate marathon and not some Nike experiment. Of course if you could run 1:39.99 for 800m it is almost guaranteed that you would be fairly competitive at either 400m or 1500m, likely at least one of them. If you can run a legitimate 1:59:59 marathon, you are obviously a great half marathon runner and almost guaranteed to be fairly competitive at 10000m. Also I will add that in this hypothetical you are the first and currently the only one who has broken either 1:40 or 2:00. Which do you choose?