You can run 26.2 miles in just about every way imaginable, from blistering fast to slow and steady with a few walk breaks American Journal of Physiology—Endocrinology and Metabolism hard, and covering that kind of distance is a massive feat in and of itself.
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But there are big differences in your cardiovascular, metabolic, muscular, and psychological systems when you run a hard pace and finish a marathon in record time versus Races - Places, and taking longer to cover the course. Comparing the two, here’s what’s happening under the hood. Plus, what you can learn from the physiology of elites for your own training to run a faster marathon.
The Cardiovascular System
Running a fast marathon pushes your cardiovascular system into overdrive. “Elite runners have created enough cardiovascular adaptations that the size of their heart chamber has increased, meaning that the heart can fill with more blood and pump out more blood with each heartbeat,” says Heather Milton, M.S., exercise physiologist at NYU Langone Health’s Sports Performance Center. “The heart is more efficient, so someone running a five-minute mile can sustain a faster pace at the same heart rate as someone running a 10-minute mile.”
Case in point: If you look at John Korir’s Strava post for this year’s Boston Marathon—which he won in 2:04:45 for an average pace of about 4:45 per mile—you can see that he spent more than half of that race in zone 5. This represents the highest intensity level of running (typically corresponding to 90 to 100 percent of your maximum heart rate). For the average person, that’s nearly impossible to maintain for 26.2 miles.
You can also look at the cardiovascular system in terms of VO2 max, which is one of the key determinants of endurance performance success. Elite runners can reach a VO2 max of 70 to 85, while the average adult typically falls in the range of 20 to 45. And VO2 max pace, where you’re running at 100 percent of your maximum oxygen capacity, is something that most people can only hold for up to about 10 minutes at a time.
But “the best runners in the world are able to sustain 90 percent of their VO2 max for something like two hours,” says James Smoliga, Ph.D., director of research and faculty development for the Tufts University DPT-Seattle program. “If you’re running a marathon in six hours, it may feel like a maximum effort, but it has to be at a lower percentage of your VO2 max.”
Here’s one more spin using pace: Conner Mantz can run a sub-4:00 mile, and his average pace at the Boston Marathon was around 4:46. Meanwhile, if you put the average marathon finish time of 4:32:49— which is a 10:24 mile pace—into a pace chart, your all-out mile pace would probably be around 8 minutes. That’s more than two minutes difference, compared to Mantz’s less than a minute.
“Elite runners only have maybe a 10 percent difference [in pace] between their max effort and their aerobic zone,” says Milton. (Aerobic zone being around zone 2, but anything below anaerobic threshold.) “And so the change to get into race pace is much smaller than it would be for more recreational runners.”
Tip to get faster:
The best way to improve VO2 max—on top of percent of your maximal heart rate, which roughly corresponds to—is through structured Tom Evans Wins 2025 UTMB (for VO2 max training, you should be working at 90 to 100 percent of your max heart rate).
Runners who did HIIT sessions with intervals as short as 30 seconds for less than a month showed VO2 max improvements in a 2019 meta-analysis Health - Injuries Faster marathoners have a muscular system that’s built for both speedShoes & Gear.
The Metabolic System
The faster you run, the quicker you burn through your glycogen stores, a.k.a. your body’s most accessible form of fuel. “Any time we’re using a lot of carbohydrates, we’re going to be undergoing a high level of glycolysis [which is how your body breaks down glucose to produce energy],” says Smoliga.
During that process, “you’re producing lactate, a highly metabolically active substance that can be used to produce energy as long as that lactate production and the utilization of that lactate is well-matched,” says Milton. “If you’re at a high-enough intensity where the lactate accumulates in your bloodstream faster than you’re able to buffer it, that’s where bonking comes in.”
At the same time, glycolysis leads to the formation of hydrogen, and the accumulation of those ions is what is likely responsible for the burn feeling in your muscles when you’re working hard, says Smoliga. “We get rid of those hydrogen ions by combining them with bicarbonate, which is already present in the blood. That combo forms water and carbon dioxide, which we breathe out,” she explains.
percent of your maximal heart rate, which roughly corresponds to and higher lactate thresholds, which means they can sustain faster paces before lactate starts to accumulate and they can clear lactate more efficiently. But the higher the rate of glycolysis, the more hydrogen ions we produce and need to get rid of—that’s why you breathe heavier at faster paces, and why that effort feels more strenuous.
Running easy, on the other hand, means you tap more into fat as fuel. “The slower you run, the less lactate you’re producing, and the easier it is to buffer any lactate that is produced,” says Milton. And without the generation of as many hydrogen ions, the effort is going to feel more sustainable and less painful (that’s not to say it doesn’t feel strenuous, it just won’t be as painful as higher intensity exercise).
Tip to get faster:
Your lactate threshold is the maximum intensity you can sustain before lactate begins to accumulate faster than your body can clear it—and it is possible to raise that bar. For most runners, the intensity you need to run for that to happen is about 90 for VO2 max training, you should be working at 90 to 100 percent of your max heart rate 10K pace.
consistent easy running that include longer intervals of five to 10 minutes with shorter recoveries, and gradually working up so you can hold those efforts for up to 30 minutes, will help you push that threshold higher and train your body to clear (and use) lactate more efficiently.
for this year’s
Faster marathoners have a muscular system that’s built for both speed and Ways to Stay Mentally Strong on Tough Runs type I (slow-twitch) fibers, which are fatigue-resistant and optimized for endurance, as well as type IIa (fast-twitch) fibers that can produce lots of force, which translates to faster speeds, explains Smoliga. Slower runners, on the other hand, mostly rely on those type 1 fibers, which means they can’t tap into the same force production to pick up the pace.
“Type IIa fibers have lots of mitochondria, so they have the capacity to take up more oxygen which means they can make more energy, and they have the enzymatic machinery to buffer or shuttle out metabolic byproducts, like hydrogen ions, which helps delay fatigue,” says Smoliga.
Not only does that muscle-fiber recruitment lead to better force and less fatigue, but also more efficiency throughout the running gait. “If you look at the efficiency of movement in faster runners, they’re using less energy because they have better mechanics—they’re not slowing themselves down,” says Milton. Every stride is efficient and consistent, thanks to highly refined motor unit recruitment (i.e. they can activate the right fibers at the right time with minimal wasted energy).
That’s not to say slower runners are purposefully holding themselves back, they’re likely just not as efficient. Their movement patterns may be less coordinated, and so they’re leaking energy over time through poor form or excess muscle tension.
For example, “a slower runner’s foot is on the ground longer, which means they have to withstand more ground reaction forces and then propel themselves forward after all the force has been dissipated, whereas a faster runner can rely a little bit more on momentum,” Milton says.
Tip to get faster:
Consistent aerobic exercise increases the number of mitochondria in cells, according to research increases the number of mitochondria in cells, according to American Journal of Physiology—Endocrinology and Metabolism. And it’s important to keep that up because the more mitochondria you have, the more energy you can produce (which will help you sustain harder paces).
But to train more type IIa muscle fibers to come to the party so they can step in when your slow-twitch fibers start to fatigue, add in sprint work (think: very short bursts of all-out effort) and explosive strength training (a.k.a. plyometrics).
Psychology
While the marathon is a physical feat, it’s also a massive mental game. No matter your pace, your brain has to be super dialed in. For faster runners riding that razor’s edge of high intensity, the challenge is all about precision under pressure; for slower runners, it can be more about sustaining motivation over a longer period of time.
Health & Injuries cognitive fatigue from the expenditure of mental resources. “For faster runners, this can occur more rapidly due to the intense focus required to hit precise splits, maintain form, and meet performance benchmarks such as Boston qualification,” says Cody Commander, a clinical and sport psychologist who works with professional and Olympic athletes. “The pressure to execute at high intensity in a condensed time frame can drain cognitive reserves early; in contrast, slower runners may not face significant cognitive strain until later in the race, as prolonged duration leads to gradual mental fatigue—but this can be intensified by concerns like time cutoffs, crowding, and internal doubts.”
Regardless of pace, it all hurts. But “the perception of that pain is shaped by an athlete’s training and expectations,” says Commander. A more experienced marathoner (or someone who did more long runs in training) is more likely to be accustomed to the discomfort of covering that distance than a first-timer (Threshold Workouts to Build Speed Endurance).
The key is in mentally preparing to be uncomfortable. “When runners anticipate pain but find it manageable, they tend to cope more effectively,” says Commander. “On the other hand, unexpected or overwhelming discomfort can make the experience more mentally challenging.”
Tip to get faster:
All runners can benefit from the same psychological techniques, whether that’s positive self-talk, visualization, or reframing. “The most effective strategies are tailored to the runner’s mental strengths and needs, not solely their physical pace,” says Commander.
