NPR had an interesting article about avoiding the infamous "Wall." (You can find the original article here.) The jist of it is that if you use your heart rate at rest and during exercise (to estimate VO2 max) combined with your weight in order to estimate your "fastest theoretical marathon time". In order to reach that pace, you would need to train properly, carb load and have "grit."
What did I think about the article? My first thought was "duh." Many of us could probably run much faster under ideal circumstances. If we push ourselves harder, have proper training and eat well we could all, likely, shave a couple (or many) minutes off our current PRs. And articles like this can help you go that extra mile. If makes you wonder how much better you could do if you made those important changes. However, those are a lot of ifs and most of us have many other factors in our lives demanding our time and attention. Sure, I could probably run 30 minutes faster if training were the focus of my days. I must admit, I do like a challenge. But I also like enjoying the races I do, and I love my work. I'm no where near wanting to dedicate myself solely to training.
What I take from the article is that we can all improve and there are many ways to do so. What are you doing achieve your next PR?