I’m sorry to hear about your disappointing race. Bonking can happen for any of a number of reasons, as you yourself have suggested. It’s something that every competitive runner experiences sooner or later, and there’s always a risk of bonking when you start a longer race at the fastest pace you feel you can sustain through the full race distance. If you miscalculate even slightly on the side of aggressiveness, either because you simply are not quite capable of the time you hope to achieve or because race conditions thwart your expectations or even because it’s just not your day, the wheels may come somewhere short of the finish line.
The good news is that pacing is an art in which all runners become more skillful with experience. Have you ever watched a kids’ fun run? You can always tell which youngsters have never raced “long distance” before, because they take off at a sprint and wind up walking before they’re halfway done. All it takes is one or two experiences like that, though, before one starts to cultivate a sense of pacing—that is, the ability to restrain one’s pace just enough to finish in the minimum time possible without faltering.
No runner ever completely masters the art of pacing, however, because no two races are ever identical, because a runner’s body is never in exactly the same state in any two races, and because the perception of effort that runners rely on to regulate their pace in races is not a perfectly precise mechanism–like an atomic clock whose information requires no subjective interpretation. But what fun would racing be if you always knew exactly how fast you were capable of running? The fact that we can always improve in (but never perfect) the art of pacing keeps racing mentally challenging as well as physically challenging.
I suspect that at a relatively early point in the Rock ‘n’ Roll Chicago Half Marathon you began to feel sensations that you would have perceived as warning signs if you’d had a little more racing experience in your legs. Effective pacing comes with knowing how you should feel at any given point of a race of a given distance. With only one past half marathon experience to go by, you may have accepted a level of discomfort two or three miles into the races that you would have interpreted as a sign that you needed to slow down if this had been your fourth or fifth half marathon.
Pacing aside, I can’t answer the question why you were not capable of running as fast in your second half marathon as you were in your first. I can tell you that it’s important to use training to develop a very good sense of the kind of race performance you are truly capable of. One way to do this is by performing challenging workouts at your goal race pace and/or at race-effort level. As you gain experience in training and racing, you can really use all of your workouts to compare your current fitness level to your fitness level before past races. Use the difference as the basis for an estimate of your performance capacity in your next race.
Never let the clock have the final say, however. Your training can give you a reasonable goal pace to take to the starting line, but to have your best possible performance in the race, you must be willing to modify your goal before you start if conditions warrant it (e.g. it’s really humid) and you must also be willing to adjust your pace as necessary during the race based on how you feel. Adjusting doesn’t always mean slowing down, mind you. One of these days you’re going to discover that you feel surprisingly good while running at your goal pace during a race, and in that circumstance you need to trust your body enough to ignore the clock, speed up, and run your way to a major breakthrough!