Race season is here. My inbox right now is half callanetics students asking about hip mobility and half running friends asking the same question in slightly different shapes: how do I not blow up at 70K?
If you have a 100K on your calendar this spring or summer — a first one, a redemption one, a "I have unfinished business with that mountain" one — this is the article I wish someone had handed me before my first.
I am going to break the race into four zones, the way I actually run it in my head. Each zone has a different job, a different failure mode, and a different relationship with pain. I will also tell you, as a physiotherapist, why your body falls apart exactly where it does — and what you can do about it before, during, and after.
Why 100K is its own animal
A marathon hurts because you go hard. A 100K hurts because you cannot. The whole craft of the distance is rationing — pace, calories, attention, willpower. You will be on your feet somewhere between 10 and 20 hours depending on terrain and ability, which means the cumulative load on your Achilles, plantar fascia, calves and knees will be roughly an order of magnitude beyond your longest training run.
That is not a problem you solve on race day. It is a problem you manage on race day, with a plan you built in training. Everything that follows assumes you have done the work — long back-to-backs, vertical, fueling rehearsals. If you have not, read my piece on building an aerobic engine for trail runners first, then come back next season.
Zone 1 (0–30K): Discipline. Do not bank time.
Every 100K wreckage I have ever witnessed — including one of my own — started here, in the first three hours, when everything felt absurdly easy.
The trap is gravity. You taper, you sleep slightly more than usual, you eat carbohydrates for three days, and then a horn goes off at 5 a.m. with a thousand other people who are also fizzing with adrenaline. Your watch beeps a pace you have never sustained for that long. You think: I feel great. I will put time in the bank now and cruise later.
There is no bank. There is only debt.
What I actually do in Zone 1:
- Run the first hour 20–30 seconds per kilometer slower than I "could." On technical trail, I walk every uphill that puts my heart rate above easy conversation. Every single one.
- Eat the first gel or piece of real food within 30 minutes. Not because I am hungry — I am not — but because my stomach still works and I want to teach it that food is arriving on a schedule.
- Drink to thirst plus a little, with electrolytes, every aid station.
- Check my form. Tall through the chest, relaxed shoulders, quick light feet. If my form is sloppy at 20K, it will be tragic at 80K. I wrote a whole article on why your running form falls apart at mile 18 — it is mostly about the gear and habits that keep mine honest.
The internal mantra here is boring on purpose: the race has not started yet.
Zone 2 (30–60K): The boring middle, where fueling errors compound
This is the dangerous one. Not because it hurts — it does not, yet — but because nothing dramatic is happening, and your attention drifts.
You are warm. You are settled. The leaders are gone, the back of the pack is gone, and you are alone with a couple of strangers on a fire road that goes on for an hour. This is exactly where people stop eating.
I have seen it in others and I have done it myself. You forget one gel. Then you "do not feel like" the next one. You skip the soup at the aid station because you want to be quick. By 55K you have a 400-calorie deficit you cannot dig out of, and the first real climb is about to hand you the bill.
The Zone 2 rules I actually obey:
If you are a Type 2 diabetic, have a sensitive gut, or know your stomach historically shuts down at hour 6, Zone 2 is where you start sipping ginger tea or eating something cold and bland before the nausea arrives. Prophylactic, not reactive.
Zone 3 (60–85K): The pain cave — what is actually happening
Welcome. Most runners arrive here somewhere between hours 7 and 10. The feeling is unmistakable: a deep, generalized ache through the calves and feet, a knee that "was not bothering me before," a low simmer of nausea, and the seductive thought that maybe you should sit down for "just a minute."
As your physiotherapist friend, let me tell you what is actually happening underneath, because once you understand it, it becomes much easier to manage.
Your tendons are losing their bounce
The Achilles and plantar fascia store and release elastic energy with every step — that is what makes running efficient. Under hours of repetitive loading, the collagen matrix gets progressively less springy. You stop bouncing and start thudding. This is why your form collapses and why every step now sends a duller, deeper signal up the chain.
If you have a history of Achilles trouble, please read my Achilles return-to-sport timeline in the off-season. Race day is not the time to test a half-rehabbed tendon.
Your calves are pump-failing
The calf muscle is your secondary venous pump — every contraction pushes blood and lymph back up your leg. After 8 hours of work, the muscle is fatigued, the pumping action weakens, and fluid pools downward. Your feet swell. Your shoes get tight. Pressure rises under the metatarsals and in the toe box, and what felt like the perfect shoe at 20K now feels like a vice.
This is the single biggest argument for calf compression on a 100K. Graduated compression supports the venous return your fatigued muscle can no longer do alone. I have raced both with and without, and the difference in the back half of the race — particularly in how my lower legs feel the next morning — is not subtle. The HYKLE Compression Socks are what I usually run in; on point-to-point races where I cannot change at drop bags, I will sometimes go with the HYKLE Ankle Compression Socks under trail gaiters when the weather is hot.
Your knee is angry because your hips are tired
The "knee pain" that shows up at 70K is almost never a knee problem. It is a glute medius problem. Your stabilizers fatigue, your pelvis drops slightly on each step, and your knee starts tracking inward under load. The kneecap gets unhappy. If this is a recurring story for you, my piece on what runners get wrong about runner's knee explains the mechanism in detail.
Your brain is making it worse than it is
Fatigue catastrophizes. Hunger catastrophizes. Dehydration catastrophizes. Before you decide you are "broken," eat 200 calories, drink 300 ml with electrolytes, sit down for two minutes, and reassess. I would estimate that 70% of the dropouts I have witnessed at hour 9 were calorie and salt problems wearing a costume.
My Zone 3 protocol
- Shorten the horizon. I stop thinking about the finish. I think about the next tree, the next switchback, the next aid station. Distance becomes time becomes a single 30-second exhale.
- Walk the steep stuff with intent. Hands on quads, drive from the hip. This is not "giving up" — at certain gradients, power-hiking is genuinely faster than running, and it spares the calves for the runnable sections ahead.
- Audit feet at the aid station. Sit. Shoes off. Sock off if anything is squelching. Re-lube, re-tape, fresh dry socks if you have them. Five minutes here saves you a DNF.
- Breathing cadence. I match my breath to my steps — 3 steps in, 2 steps out on flats; 2 in, 2 out on climbs. It quiets the panic and rations effort.
Zone 4 (85–100K): The final push
You made it through the cave. Something strange happens here: the suffering stops being acute and starts being weirdly tolerable. You can see the finish in your mind. The job now is not to do anything stupid in the last 15K.
Chunking. I do not think about "15K to go." I think: 3K to the aid station. 4K to that ridge. 2K of downhill I can shuffle. Then a parkrun home. Big numbers crush morale. Small numbers feel doable.
When walking is faster. On any uphill steeper than maybe 8–10%, late in the race, brisk walking with good arm drive will match or beat a "run" that is really a shuffle, and it will cost you a fraction of the energy. Use this. There is no medal for running the unrunnable.
Save something for the descents. This is where you will overtake people. Late-race downhills wreck quads that have not been trained for them, so other runners will be tentative. If your quads have anything left, this is where you spend it.
The final 3K. Heart open, eyes up, breathe. You earned this. Whatever the clock says, you covered 100 kilometers on your own two feet.
What I am wearing and packing
A short, honest list. I will not pretend gear wins races — it does not — but the wrong gear can absolutely lose one.
- Calf compression from the start, not "if I need it later." See above on the venous pump argument. HYKLE Compression Socks are the pair I run in. Customers who stand all day in healthcare and retail tell us the same thing I notice in the back half of an ultra — the legs simply feel less wrecked at the end.
- Two pairs of socks in the drop bag. Dry socks at 50K and 80K are a luxury that costs almost nothing and pays back enormously.
- A half-size-larger backup pair of shoes in a late drop bag if your race allows it. Feet swell. Knowing fresh, roomier shoes are waiting at 65K can be a mental lifeline.
- Salt, salt, salt. I carry more than I think I need.
- A pacing band on my wrist with target splits, fueling reminders, and a one-word mantra. Mine usually says patient.
If you want a more comprehensive packing list — what goes in vest, drop bag, crew bag — I broke it down in my long-run gear checklist for anything over 20K.
A personal note
My worst 100K — the one I think about most — was a race in the Rhodope mountains a few years back where I executed Zone 1 perfectly, ignored Zone 2 completely because I was chatting with a friend who pulled ahead, and walked into Zone 3 with a 600-calorie hole and dead legs. I sat in an aid station chair at 68K and seriously considered dropping. Deso was crewing. He did not give a speech. He handed me hot broth, a boiled potato with salt, a fresh pair of socks, and said "twenty minutes, then you decide."
In twenty minutes I was a different person. I finished. Slowly, ugly, but I finished.
That is the lesson under every other lesson in this article: the pain cave is almost never as terminal as it feels in the moment. Fuel, sit, change something small, and reassess. The race is long enough that you get to be a new version of yourself several times before the finish.
Run smart. Eat early. Compress your calves. See you at the start line.