I have observed a pace group divide: accuracy decreases with target time increase. The pace groups that approach 3 hours come closer than the pace groups that approach four hours and beyond. I have witnessed a 4:15 pace group come in close to 4:30, while the 3:10 pace group was pretty much dead on. I imagine the difference is that there are more experienced runners at the faster paces. No matter. It wasn't the pace group's marathon to run for you.
A marathon is a race. Run your own. Yes, I know that there is a popular movement to "just finish" a marathon, but let's face it: If you're capable of finishing a marathon, you're capable of racing a marathon.
If you're going to race, you have to maximize your performance. Now, most of that comes long before race day. You should have been training for marathon race day in mind for at least the last 16 weeks... Maximizing your performance means listening to your body. Part of your training for the marathon should have been toward understanding what your body is telling you. It's good to practice this at progressively longer distances: 5k, 10k, 15k/10mi, 13.1 miles... Each one of these distances has a different feel in terms of the speed and fatigue factors.
To paraphrase some pacing advice I received:
On race day, start out by running at a pace that you believe you can sustain for 26.2 miles [forget what your Garmin is telling you]. Constantly listen to your body, the conditions, and the terrain. At mile 20, if you've run the race properly, doubt and some serious fatigue will set in. At that point, run like hell until you finish.