My biggest fear is one day, not being able to get any faster. So what truly is the difference between the 3:5x milers and the 4:1x milers? How far can you push before genetics stop you? If I give it my all through high school and I cant run under 4:20 is there anything I will ever be able to do to become great. What if you cant push any more? If there is a real answer to this question or advice I would like to hear it please.