Your training sounds much like that training of many high school freshmen. Your time would be slightly above average for them, but you are ten years older, so I would guess your results were about average.
The average high schooler who can run 18:30 as a freshman runs in the 16s by senior year. Of course, average means everyone is either faster or slower than the average, with the 16s themselves being a lot of difference from 16:00 to 16:59, and others slower or faster than that.
Potential is a different question. Run harder, smarter, longer and likely you, as most any 24 year old male, could get in the 15s before you are old enough to slow down from age. Your 18:30 after four months suggests that you don't have potential to break 13:30 and likely not break 14:30. Typically, people who do, have prodigous results by the time they've been running modest miles four months (Alan Webb and Craig Miller come to mind, contending for first place in very competitive varsity races by then), already in the 15s by that point. Also talented, but less rare people, are usually in the 16s after four months. Was the 18:30 because of you not trying harder, training incorrectly, overweight, etc? Unknown, but 18:30 on your time and volume isn't prodigy level, just good.
Your direct question is unanswerable, as people are different. Of those different people, which one are you? Who knows? With your speed, and a year of 50-70 mile weeks you might be in the 15s, or you could still be setting a goal to get to 16:59.
Either way, the effort to get into better and better shape is a very worthy one.