You’ve probably felt it before: mile 14 feels manageable, maybe even comfortable, but somewhere around mile 17, everything changes.
Your legs suddenly feel like they’re encased in concrete.
Each step requires conscious effort where before you were gliding.
That mental conversation starts: “Maybe I should walk,” “I still have 3 more miles,” “Why did I sign up for this?”
If you’ve experienced this brutal shift, where what felt manageable suddenly becomes a survival march, you’re not alone and you’re definitely not undertrained.
Research by Benjamin Rapoport at MIT [1] demonstrates that more than two-fifths of marathon runners experience “hitting the wall,” with athletes running at 80-95% VO2max depleting glycogen most commonly around mile 21.
That’s not surprising if you’ve been running long enough, this brutal reality of miles 16-20 is where marathons are won or lost.
But here’s the good news: the difference between limping to the finish with a 15-minute positive split versus maintaining your pace through those final miles isn’t about toughness or talent.
It’s about understanding what’s actually happening in your body and implementing proven strategies to prevent metabolic failure before it starts.
Let’s look at the science behind why these miles feel exponentially harder, and more importantly, exactly how to fix it.
The Science Behind Why It Gets Exponentially Harder
Your body stores enough glycogen to fuel approximately 90-120 minutes of running at marathon intensity.
For most runners, this means the tank starts approaching empty somewhere between miles 16-20, depending on pace and individual physiology.
A computational study [2] demonstrated that athletes with various leg muscle builds and glycogen densities, when running at 80-95% VO2max, hit glycogen depletion around mile 21, the distance empirically identified as where marathoners most commonly “hit the wall.”
But glycogen depletion isn’t the only villain in this story.
Cardiovascular drift progressively increases your heart rate while maintaining the same pace, driven by dehydration and rising core temperature.
Research on negative splits [3] shows that starting conservatively reduces early cardiac stress and delays the rise in heart rate, maintaining more stable cardiac output over time.
Your brain also gets involved through central fatigue, increasing serotonin levels that reduce neural drive to recruit muscle fibers, creating the sensation of extreme fatigue even when your muscles could theoretically continue.
Simply put: multiple systems are failing simultaneously, which is why discomfort at mile 10 becomes agony at mile 18.
The Fueling Strategy That Saves Your Back Half
The biggest mistake runners make is starting their fueling too late.
Studies show [4] that beginning carbohydrate intake at 30-45 minutes into your run, around mile 3-4, prevents the sudden energy crash that derails so many long runs.
Target 30-60 grams of carbohydrates per hour for any run exceeding 90 minutes.
Take small, frequent doses of 8-15 grams at a time rather than overwhelming your gut with large amounts.
This approach, backed by research on GI distress [5], allows your digestive system to absorb nutrients efficiently without causing cramping or runner’s diarrhea.
Always pair concentrated carbs like gels and chews with water, never with sports drinks.
Combining concentrated sugars increases the risk of gut distress and those dreaded bathroom emergencies.
Your pre-run fueling matters just as much as what you consume during the run.
Eat your final meal 2-3 hours before your long run, focusing on easily digestible carbs while avoiding high-fiber, high-fat, or high-protein foods that take longer to digest.
Research on GI management [6] shows that reducing fiber and fat intake 1-2 days before helps prevent mid-run bathroom emergencies that can derail your entire effort.
The Pacing Discipline That Makes or Breaks You
Here’s where most runners fail: they start too fast because they feel too good.
Analyses of world-class marathon events show that a majority of record-breaking performances follow either an even or slight negative split profile.
Starting conservatively allows for more gradual glycogen utilization, preserving energy reserves for the final stages of the race.
Your first 5 miles should feel almost boringly easy, this is not the time to prove anything.
A study on marathon pacing [7] found that 92% of marathoners fail to achieve negative splits not because it’s impossible, but because they lack the preparation and discipline required.
Position yourself with runners 10-15 seconds per mile slower than your goal pace at the start.
Focus on effort over pace in these early miles, resisting the temptation to go out hard when you’re feeling fresh.
The 10-10-10 method provides a practical framework: run the first 10 miles conservatively (10-15 seconds slower than goal pace), settle into goal pace for miles 11-20, then push hard in the final 10K if you’ve fueled and paced correctly.
By starting at a more moderate pace, research shows you reduce early strain on the heart and delay cardiovascular drift, maintaining more efficient oxygen transport to working muscles.
Mental Strategies for When Your Body Wants to Quit
Breaking down 20 miles into smaller, manageable segments makes the distance feel psychologically achievable.
Research on endurance psychology [8] demonstrates that “chunking”, mentally dividing a race into segments with specific goals for each, increases athletes’ perceptions of control.
Try thinking of a marathon as 6 x 7K runs, or 4 x 5-mile blocks, or even 3 x 40-minute efforts.
Set mini-goals and rewards for each chunk: “I’ll take my next gel at mile 12,” or “I’ll reassess how I feel at the next water station.”
Studies investigating brain strategies [9] found that external dissociation, focusing on scenery, crowds, and supporter signs rather than your discomfort, appears most effective for non-elite runners and results in later onset of fatigue.
Mental arithmetic also provides powerful distraction: counting houses, calculating remaining distance, or tallying up pace splits keeps your mind engaged when it wants to focus on how much everything hurts.
Develop instructional mantras that keep your form sharp when fatigue sets in.
Short, rhythmic phrases like “head up,” “strong finish,” or “I’ve trained for this” override doubt and fatigue while maintaining steady stride.
Research on self-talk [10] shows that positive affirmations increase self-regulatory resources and enhance decision-making during difficult moments.
Solving the Bathroom Emergency Problem
Up to 90% of endurance runners experience some form of GI distress during training or racing.
Running diverts blood from your GI tract to your working muscles (splanchnic hypoperfusion), which irritates the digestive system and triggers your body to eliminate.
The mechanical jarring of running, combined with stress hormones like cortisol and adrenaline, creates the perfect storm for bathroom emergencies.
Prevention starts 72 hours before your long run.
Research on pre-race nutrition [11] shows that reducing high-fiber foods in the days before prevents the sudden urge to go mid-run.
Wake up early enough to allow your body to naturally eliminate before you start running, this simple strategy prevents countless mid-run pit stops.
Morning coffee or tea can help regulate bowel movements, but consume it at least an hour before running to give yourself time to use the bathroom.
During the run, if GI distress strikes, immediately slow your pace, reducing intensity decreases the severity of splanchnic hypoperfusion and often provides quick relief.
Walking briefly reduces gut motility and decreases urgency for most runners.
For runners who consistently struggle despite proper nutrition and timing, over-the-counter anti-diarrhea medication taken the evening before and race morning can help control the situation.
Just experiment with medication protocols during training runs, never on race day.
The Reality: It’s Supposed to Hurt
Let’s be honest about what you’re signing up for.
Miles 16-20 will never feel “easy” for anyone, including elite runners who simply suffer faster than the rest of us.
What separates successful long runs from catastrophic ones isn’t the absence of discomfort, it’s the difference between managed difficulty and complete metabolic failure.
The physiological reality is that running 20+ miles pushes multiple body systems to their limits: glycogen stores deplete, cardiovascular efficiency decreases, core temperature rises, and neuromuscular fatigue accumulates.
Understanding that this difficulty is normal and expected, not a sign that you’re undertrained or weak, helps you push through when every step feels impossible.
Your brain creates fatigue before you reach true physiological limits, a protective mechanism that tries to prevent damage.
Research on the central governor theory suggests you always have more in the tank than you think, which means the voice telling you to stop at mile 18 is often lying.
Putting It All Together
Start your long run preparation 72 hours before by reducing fiber and fat intake.
Eat your final substantial meal 2-3 hours before the run, wake early enough for natural bathroom elimination, and begin hydrating 30 minutes before you start.
In the first 10 miles, run conservatively even though you feel great, start fueling at mile 3-4, take small frequent sips of water, and practice chunking by focusing only on the current segment.
From miles 11-16, settle into your goal pace if running even splits, continue consistent fueling every 30-45 minutes, and maintain positive self-talk as difficulty increases.
When you hit miles 16-20, expect discomfort and recognize it as normal and manageable.
Focus on one mile at a time, maintain your fueling even if you don’t feel like eating, and use mantras aggressively.
If you’ve paced and fueled correctly, this is actually where you can push rather than just survive.
Remember: every runner who’s crossed a marathon finish line has survived these miles.
You’re not weak for struggling, you’re experiencing exactly what the distance demands.
