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2026-05-25 · 4 min read


The marathon wall


If you've run a marathon, you've probably hit it. If you haven't, it's likely because you ran fewer km than you thought or the plan was built well. The wall isn't a myth or mental weakness: it's glycogen depletion with a couple of extra complications.


What happens in the body when the marathoner "dies"


Your primary engine in a 3-4 hour race is muscle glycogen. When that stock falls near zero:


1. Fast fuel drops out: your body tries to use fat, but fat yields fewer kcal per minute. Result: pace drops 30-90 s/km without RPE feeling easier — the opposite.

2. HR climbs at the same target pace: the body works harder to produce less power.

3. The brain runs low on glucose: mild hypoglycemia → confusion, bad decisions, glassy eyes, that "I'm dreaming" feeling.

4. Cumulative muscle microdamage: type II fibers break down, type I fibers arrive saturated. Quads and calves start cramping.


This typically hits between km 30 and 35 (mile 19-22). It's not always a sudden crash; often it's a gradual fade where pace slowly bleeds away.


Why it happens (the real causes)


### 1. Going out faster than your fitness can hold


Mistake #1. You feel good at km 5 (everyone feels good at km 5), you take it 5-10 s/km faster than goal. Multiply by 42 km. You pay the bill from km 25 onwards.


### 2. Not eating enough during the race


If you hit km 25 with 0 g of extra ingested glycogen — wall guaranteed. You need 40-90 g CHO/h during the race (depending on how you trained the gut, see the [carbs in ultra](/blog/carbs-durante-ultra) article).


### 3. Not training marathon-specific long runs


A 20 km long run doesn't prep you for a marathon. You need:

  • Long runs of 30-32 km with last km at goal pace.
  • Long runs with blocks at marathon pace (3x5 km at M pace).
  • At least one 3+ hour long run so the body learns to use fuel at that duration.

    • ### 4. Random pacing: too fast on climbs, brake on descents


    Smart marathon pacing is: slightly slower on climbs (don't claw back ground for ego), cruising speed on descents (don't accelerate like a cross country race). Marathons punish effort variance.


    ### 5. Underestimating heat + humidity


    At 77°F (25°C) your marathon pace target becomes 4-8% slower than at 55°F (12°C). If you don't adjust the goal when the day is hot, you'll wall earlier (see the [temperature calculator](/herramientas/temperatura)).


    How to dodge it


    ### In training (weeks before)


  • Progressive long runs: 28, 30, 32 km in weeks -6 to -4. Last 8-10 km at target marathon pace. Trains the body to produce energy when fatigued.

  • One or two fast-finish runs: 24-28 km where the last 5-8 km are at goal pace or a touch faster. Simulates the real scenario: start well, finish accelerating.

  • Threshold sessions near M pace: 3x15 min or 4x10 min at threshold or slightly above M pace. Pushes your LT2 (second lactate threshold) and moves the "aerobic ceiling" up.

  • Weekly total volume: for sub-3:30, ~60-70 km/week sustained in the 8 peak weeks. For sub-4:00, 45-60 km/week. Without volume there's no base.

    • ### In nutrition pre + during


    Carb load (48-72h before): 7-10 g CHO/kg/day. Not stuffing yourself — prioritizing pasta/rice/potato/bread in every meal, dropping protein and fat. You arrive with maximum glycogen.


    Race-day breakfast (3-4h before): 100-150 g CHO with little fat/fiber. Example: 2 slices of toast with honey + banana + 200 ml sports drink. Test it in training before.


    During the race: 40-70 g CHO/h from km 5 (don't wait for hunger). Sample plan:

  • km 8, 16, 24, 32, 38: gel.
  • Water at every aid station, sports drink alternating with water.
  • At km 32 (when the classic wall hits), a caffeine gel (50-100 mg) gives you neural push.

    • ### In pacing


    Build your pacing plan as a range for each km, not a single number. Example for sub-3:30 (5:00/km target):

  • km 1-10: 5:02-5:05 (conservative).
  • km 11-21: 4:58-5:02 (goal pace).
  • km 22-32: 4:58-5:00 (hold).
  • km 33-42: 4:55-5:05 (whatever the body gives).

    • If km 5 reads 4:50, slow down deliberately. Start-line adrenaline lies to you.


    And if you wall anyway?


    If you hit km 33 with the wall already there:


    1. Walk 1 min at every aid station. Recovers heart rate and lets some fuel in.

    2. Caffeine gel NOW. If you didn't try caffeine before, this isn't the moment.

    3. Drop pace to sustainable even if your ego hurts. Better to finish 10 min slower than DNF.

    4. Talk to yourself. "Almost there. One more km. Then another." Slice the distance into micro-chunks.

    5. Stop watching the watch: if the goal fell apart, look at feet + breathing. Re-set pace from inside.


    Bottom line: the wall is prevented, not survived


    The most common error is thinking you can mentally muscle through. No. The wall is metabolic. What gets trained with willpower is prevention — quality km, rehearsed nutrition, conservative pacing in the first 10 km.


    Stick to these points and on race day you'll reach km 35 with enough energy to run the last 7 km as your best version of the day, instead of just surviving.