When running any distance, the bonk is bound to happen…it happens all the time. If you’ve ever been crushing it through the race, and then, out of nowhere, your day turns upside down to feeling tired, slow, heavy, or grumpy, you’ve bonked! It is possible to train to overcome it though! We’ll try to help you understand what is happening, and how to overcome it so that you can race more efficiently at your next big day.
The Bonk may be an evolutionary protection that the body puts in to play after the brain realizes it doesn’t have adequate resources to continue at the current pace for the distance left to cover. It is a physical and mental state in which desire to move forward is low.
An ultramarathon runner will deplete a majority of their glycogen (fast energy) stores in about 140-215 minutes (2h20m - 3h35m). This hypoglycemia can result in the body lacking energy as it starts to convert other slower energy stores. 
At about this time, you may start noticing feeling of fatigue, slowness, or mental fogginess. This is a signal from the brain that it perceives the activity as a threat. Like many things, you can train for this moment to mitigate the effects of bonking and be able to press on in the race without losing pace.
Training to have less chance of bonking.
Fasted/No carb runs.
You may try to run two to three times per month in a carb/sugar deprived state. It will train your brain to realize that it is still possible to go on without a full tank, and it will begin to more efficiently use other forms of fuel, like fat. You can do these runs by running in the morning before breakfast, or after work before eating dinner in the evening. Carbs still play a role in a diet, so note this does not mean permanently cutting out carbs. Carry a gel or form of fast carbs just in case.
Run a mid-week longer run. Adding an extra 30-45 minutes (smartly) can help your body adapt to longer distances. Time on feet is more important than fast miles here.
Strategically Carb-Load. Noakes suggests that beginning to eat a carb-dominant diet seven days before a race will allow you body adequate time to build up its glycogen stores.  This longer length of time will also prevent runners from feeling they need to eat all the carbs the day before the race, which can lead to a heavy and uncomfortable stomach on race day. We’d add that it is important to eat a clean and whole-food diet at this point (and always).
Run a practice race or two. It doesn’t have to be the full distance of your target race. A half marathon could be a good tune up for a 50k, or a marathon for a 50 mile. This will train your brain and body what to expect during a hard effort with the pressures that race day brings. Complete your practice races four to six weeks before your goal race.
To overcome the bonk during a race, implement the following strategies:
Fuel up. This includes electrolytes. Some common aid station foods are cola, gels, and waffles, and cookies, all of which offer fast carbs.
Try some caffeine. Some is key, as too much can have a negative effect on digestion and be inflammatory.
Add music. After all - part of bonking is mental. Music can help change your mood and your stride!
Have you ever bonked in a big race? What strategies did you use to overcome it? Leave a comment below!
 Tim, Noakes. Lore of Running. 3rd ed., Leisure Press, 1991, pp. 68-83.