I'm not a very talented runner. With some training I ran close to 38 minutes for the 10k. In the slightly larger local races I would always finish in the top 30 or so, but never anywhere near the front. My first marathon took 3:40-ish, got it down to 2:59 with decent training. Couldn't break 3:10 for a few years. At this point I was a runner for maybe 10 years and figured I'd reached my natural plateau.
I then discovered high mileage (read Pfitzinger). I ran 33 minutes in a 10k time trial and a 2:36 solo marathon time trial this spring, after a few years of very dedicated training, slowly building up the mileage and then just putting in lots of 90-100mi weeks with 2-3 quality sessions each week. Not sure where my limits are, but I think I'm getting pretty close. I believe I can break 2:30, but only if everything aligns just well. I doubt 2:25 is realistic.
I raced a HM a few weeks ago (won it), and talked to the guys who finished behind me. They ran 1:18-1:19, and did it off of low mileage (50k a week). I couldn't even break 1:22 off of 80k a week. So I consider myself someone with little talent, I only got to my current level because of all the hard work.