Ok, here's the thing. Sprinting and distance running utilize two different systems and muscle groups. Sprinting (400m and under) is almost entirely anaerobic and uses fast-twitch muscles. Your success in this department is largely decided by genetics. For example, I am severely lacking in fast-twitch muscle fibers. I could train for years and never break 12 seconds for a 100m. Distance running, however, is completely aerobic, meaning it uses oxygen to fuel your muscles as opposed to making them use "emergency" energy that is only useful for short sprints. What this means is that everybody has the capacity to improve immensely when it comes to distance running. All it takes is lots of hard work and time.
Your aerobic capacity is not encumbered by the percentage of ST/FT muscle fibers or anything like that. With enough training, you can increase the amount of oxygen circulated to your muscles 1000 times over. Everybody still does have a limit, but the limit for aerobic work is much higher than for sprinting. What this means is that you could start out running a mile in 15 minutes, but if you were dedicated enough and wanted it enough to run 70+ miles a week, you will improve. As you seem to have found out though, after a while, you start to plateau. Kicking up the quantity and quality of your workouts would keep your improvement curve alive a while longer, but eventually you would reach your limit.
So, with that said, anybody can improve a good amount when it comes to distance running because your aerobic capacity is affected by amount of work and the quality of that work, but yes, some people are still not going to be "elite" or anywhere near that, no matter how hard they train. I am actually a pretty good example of this. I was "decent" in high school, meaning I could run a 5K in under 20 minutes. I have since been logging 80+ miles a week, and last week I ran 16:42. If I ran 100mpw I might be able to break 16. But elite is 14 minutes. There is no way I could ever get to that level, no matter how hard I trained. So while I have improved to a level that might be considered "pretty good," I am still far from being "really good" or "great."