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Marathon Training: 3 Ways to Never Bonk Again

The mathematical formula to optimize endurance and performance during your race.
Marathon Training: 3 Ways to Never Bonk Again
Andrew Cutraro

During the 2005 New York City Marathon, Benjamin Rapoport crashed hard—his legs were racked with pain and his stomach was a mess of cramps. His muscles had run out of fuel—glycogen—and were now burning fat alone. Technically speaking, he bonked. It’s a predictable phase of long-distance running. (In fact, even pro endurance athletes crash sometimes.) “You start to slip,” he says, “and then you’re gone.” Rapoport, who has a Ph.D. in electrical engineering from MIT and an M.D. from Harvard, decided to study the phenomenon and made a few surprising discoveries. “No one had developed a mathematical way of determining—based on an individual’s biometric characteristics—how to optimize performance with respect to pacing and carb loading,” he says. So Rapoport created an online endurance calculator for doing just that. By entering things like age, weight, and resting heart rate, anyone can have access to these numbers. But Rapoport’s discoveries also offer a new way to approach endurance training in general. These are his tips for finishing strong.


Most training guides suggest that runners eat along the way in order to keep up their calories, but Rapoport says that what you eat 12 to 36 hours before your run is what really provides your fuel. That’s the time to glycogen-pack your muscles and liver. 

The goal is to avoid eating during a marathon so your blood flow can stay focused on fueling your muscles, not operating your stomach. (One or two GU gels around the halfway point are OK.) Use simple starches that go down easily, like rice or pasta, and lots of fiber. (For Rapoport’s calorie calculator, check out

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Conventional wisdom tells you to replace the fluids you sweat out during a race, but Rapoport believes running on a bit of a deficit has benefits. “It can be safe and even advantageous to get slightly dehydrated during a race,” he says. “To an extent, a slight loss of water weight can enable your body to run with greater efficiency, as its metabolic engine is driving a lighter load.”

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Dean of distance running Hal Higdon, whose training program is used by thousands of marathoners every year, advocates running anywhere from one to two minutes per mile slower than your actual race pace during long training runs.

Rapoport disagrees, arguing that you should try to maintain your marathon pace on at least one of your 20-plus-mile training runs. If you’re going to hit the wall, it will happen on a long run, so condition yourself against the bonk. 

“I treat the long runs as a dress rehearsal, though I’m not as maniacal and calculated about everything as I am before a race,” Rapoport says. “The Friday before a Sunday run, I think about my diet. I make sure I eat foods that won’t give me trouble when I’m out for a run.” 

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