When it comes to giving my athletes predicted- or goal-times for any given race, there are a myriad of factors to consider. Above all, I use the detailed chart found in the book Daniel's Running Formula and it remains the foundation of how I give predicted times, paired with a dash of the art of coaching. The Daniel's Formula is perhaps the most trusted source, so why try to reinvent the wheel? The man certainly did more than his share of homework in putting it all together for the rest of the running world. However, the art of coaching that I sometimes (not always) insert is based on other factors the chart does not consider, most importantly the specificity of the training. This principle leads to the marathon distance. With any other race distance, we can usually assume the chart to be accurate because the general distance of your training runs will bring you to the start line in good shape from 5k through half marathon. However, because marathons require the greatest specificity of training (along with racing 1-mile or shorter), they usually have the greatest fluctuation in predicted times (as you might predict).
There are many more variables to consider at the marathon distance that don't show up on the radar at shorter distances, such as a breakdown in running form, whether it be a shorter stride and/or poor posture at the end of the race. When considering muscular endurance, how much strength training have you done? Fueling and/or stomach distress also become variables at the marathon distance and not at the shorter distances. Additionally, weather becomes more of a factor as distance increases. In sum, be careful when extrapolating data from short-distance races into the marathon. Even a Half-Marathon can mislead a runner. For instance, if you ran 1:40:00 in a half marathon, then your predicted marathon time would be ~3:28:00; however, this assumes you are marathon-ready. If your longest training run has only been 10 miles and you don't do any strength training for your glutes, hips, and hamstrings, then I wouldn't predict you to finish near 3:28:00 at your upcoming marathon, regardless of your recent half marathon performance.
Each of the methods mentioned in the first paragraph has its merits when attempting to predict a marathon finish time, but they also have their limitations. Specificity of training must be very high for these predictions to actually work, which means doing long runs at the goal pace and high volume training at specific points in the year. Lo and behold, the beauty of having a coach to help guide you in these directions.