You can predict your marathon just trying to run 800 meters. Yes, it sounds pretty crazy, but it sometimes works. It doesn't mean that you just try to run those 800 meters and that's all. You still have to run a lot. Remember, marathon is 26,2 miles.
So how to do it? Try to run 800 meters at least 4 times. It's the best if you do it for 10 times. And it's much better to do this in a stadion, because you know the exact distance. When you run those 800 meters always look how fast you do it. Then run slowly for the same time as it took you to run 800 meters. After all such 800 meters get an average of all times. So if it's 3 minutes, it means that you are able to run marathon in 3 hours. If it's 2min50seconds - marathon in 2h50minutes.
But your marathon time can still vary from 10 to even 50 minutes. Why? Marathon is 26,2 miles. A lot of things can happen. You don't have any rest, you have to run all the time. Maybe you feel bad that day, maybe you have pains in your legs, stomach or head. It's pretty hard to predict such things. But still, Yasso 800 is very interesting system. It maybe can't predict the exact time, but you can try to run the same time as the system says. And it's very good for your running training, doesn't matter if you are not training for a marathon.