I talked about this in the thread which discusses being competitive if you didn't run in High School/College. Check that out.
That said just off the top of my head (granted I don't know what your frame/skeletal muscle structure looks like). Your never going to run 31:00 at 180lb's. I'm 6'1' 138-142lb's and run 31 flat for 10km. So I'd watch my nutrition if I were you and let the miles drop any extra non essential or muscle mass you may be carrying around.
I will say after 18 months of training you should have some idea of what your potential may be, but only if your training correctly. What type of "club" are you with? Does the head coach have a good grasp on training philosophies or is he just someone who lends a hand and gives advice to the best of his knowledge?
For a point of reference on 18 months of training in May 2011 I ran 41:56 for my first 10km and October 2012 I ran 33:11. Granted I did train fairly smartly and was at my ideal racing weight. (The progress slows down however as I'm now generally dropping about a minute/season off my 10km rather then that first fitness jump ha. I'm hoping to run under 30:30 next April on the roads then take a crack at a decent 10000m on the track.
Give it time and make sure your maximizing your training.