I'm on here preaching about it all the time. It's about speed. Although that is largely genetic, you can still make some large speed gains at your age as opposed to later when it will next to impossible. That should be your focus. If you're a senior and you can only run a 55 second 400 meter, you will never run 4:04. Realistically, you're going to have to have 50 second 400 meter speed. That means you will have to have 23 second 200 speed which means you will need to have 11.5 100 meter speed. Now this is all give or take a little here or there but you get the idea. So if you want to run a 4:04 mile, your goal should be to put down a quick 100, then 200, then 400 and then we can talk about the mile. Without the requisite speed, it isn't going to happen. At your age you need to start at the short end. There are far too many people on here preaching mileage. That's not going to be the answer for guys your age.