LSD (Long Slow Distance): the Zone 2 long run

LSD: the run everyone goes too fast on

LSD, for Long Slow Distance. The term was born in the late 1960s among American runners, and the idea hasn't aged a day: run, ride or walk for a long time, slowly, below your aerobic threshold. Today we'd rather say Zone 2 long run.

It's not spectacular. It doesn't flatter the ego. It doesn't make for great stories. And yet this is where you build the endurance that holds up when the hours pile on.

The problem is that most athletes get it wrong in two ways: they go too fast, and they eat as if they were racing. This article explains why both undermine what the session is supposed to build.

What exactly is Zone 2?

It all starts from one marker: the aerobic threshold (AeT), close to what is called the first ventilatory threshold (VT1). It's the intensity above which the body starts to rely more on carbohydrates and to accumulate fatigue.

At Uphill Athlete, coach Scott Johnston defines the zones based on this threshold:

• Zone 1: from AeT −20% to AeT −10%. Very easy. You could tell a whole story without catching your breath.
• Zone 2: from AeT −10% up to AeT. You can hold a conversation, but you catch your breath every four or five sentences.

Zone 2 isn't "slow". It's controlled. You breathe, you talk, you move forward, and you could keep going for a long time. That's exactly what makes it uncomfortable: it forces you to ease off when your ego wants to speed up.

The size of the engine: two athletes, two thresholds

The aerobic threshold is easy to read on a curve: it's the point where fat and carbohydrates each supply 50% of the energy. To the left, fat dominates: the effort is sustainable. To the right, carbs take over: the tank empties and fatigue sets in.

Look where this point falls for two very different athletes:

• Too much HIIT. An athlete trained mostly at high intensity switches to carbs from 56% of max HR. An easy jog already costs them sugar.
• Aerobic base. An elite Norwegian cross-country skier stays mostly on fat up to 88% of max HR. He can hold almost his entire effort range for hours.

What separates these two curves isn't talent. It's training. Hundreds of hours below the aerobic threshold develop the mitochondria and capillaries in the slow-twitch fibres, the ones that burn fat. The threshold shifts to the right. Too much intensity, on the other hand, mainly trains the glycolytic system and leaves the aerobic engine underdeveloped.

A bigger engine. More endurance. More freedom.

Aerobic deficiency syndrome: the 10% test

Uphill Athlete has a name for the curve on the left: aerobic deficiency syndrome. It shows up when you spend most of your time in Zone 3 and above, at the expense of easy aerobic work.

The test is simple: divide your heart rate at the anaerobic threshold (AnT) by your heart rate at the aerobic threshold (AeT).

• Gap greater than 10% (for example AeT 128, AnT 150: 150 ÷ 128 = 1.17): your aerobic base is insufficient. Prioritise Zone 1 and Zone 2 before adding intensity.
• Gap of 10% or less: you've "earned the right" to add Zone 3 and 4 work.

How long does it take to fix? According to Uphill Athlete, with less than 3 h a week, count on at least 6 months. With 5 to 6 h, a clear improvement in 3 to 4 months. With 8 h or more, major progress in 2 to 3 months. There's no shortcut: only a large volume below the aerobic threshold builds a bigger engine.

Why no sugary gels on a long slow run

This is where nutrition comes in, and where most athletes unknowingly sabotage their LSD.

The goal of a Zone 2 session isn't to maximise available energy. It's to teach the body to use its own. And what you eat during exercise changes the fuel the body chooses.

• Carbohydrates slow down fat utilisation. As Uphill Athlete points out: fat oxidation decreases when you consume carbohydrates before, during and after exercise. Every sugary gel triggers insulin, and insulin slows the release of fat. The body switches back to sugar, precisely during the session meant to teach it the opposite.
• The fat tank is huge; the carb tank is not. Our glycogen stores hold roughly 1,500 to 3,000 kcal. Our fat stores, roughly 50,000 to 150,000 kcal (at 10 to 30% body fat). In Zone 2, you draw on the big tank. No need to fill the small one.
• Needing gels is a symptom. Uphill Athlete describes an underdeveloped aerobic engine as one that forces you to refuel frequently, "a gel an hour or more". If you can't last 2 h in Zone 2 without a gel every hour, it's not a gel problem. It's an engine problem.

Careful: this doesn't mean training fasted or banning carbs. Uphill Athlete recommends neither fasted training nor low-carb diets, and points out that carbs during exercise delay fatigue. We agree. The question isn't "carbs or no carbs". It's how much, and for what intensity.

A standard gel alone delivers 20 to 25 g of carbs. On a long slow run, that's already the top end of what you need in an hour. Our guidelines:

Type of effort	Carbs per hour	BSE fuel
LSD, Zone 2 long run	5 to 20 g	Zone 2
Tempo, threshold, strength	20 to 45 g	Tempo
Ultra, long race	45 to 90 g	Ultra (coming soon)

On a long slow run, you eat the way the body burns: mostly fat, a little salt, and few carbs, with no sugar spike. Enough to avoid an energy deficit on multi-hour sessions, without cutting off the adaptation the session is meant to produce.

That's why we designed the Zone 2 energy purée: macadamia, salt and 7 g of MCT per pouch, no added sugar. One pouch every 2 hours. Not to replace the gel on race day. To fuel the hours that build the engine.

Find your Zone 2

Your zones depend on your thresholds, not your age. Enter them below.

 

The most reliable method without a lab: the heart rate drift test

This is the protocol recommended by Uphill Athlete to find your aerobic threshold:

1. Warm up for about 15 minutes, then gradually build to a pace where you can still talk in full sentences.
2. Hold a constant pace (or power) for 60 minutes, on flat ground or an indoor trainer. 40 minutes minimum.
3. Compare the average heart rate of the 1st and 2nd half.

• Drift of 0 to 3.5%: you were below your threshold. Repeat with a starting HR 5 bpm higher.
• Drift of 3.5 to 5%: your starting HR is your aerobic threshold, the top of your Zone 2.
• Drift above 5%: you were above it. Repeat lower.

How much LSD per week?

For Uphill Athlete, most of your endurance volume should be genuinely easy, in Zones 1 and 2, with sessions of 45 minutes or more that progressively build toward the duration your goal demands.

The key is consistency. Aerobic adaptations come from frequent, sustainable sessions repeated for months, not from a few heroic outings. A single long run on Sunday isn't enough: aim for several sessions a week, including one longer one.

Classic LSD mistakes

• Living in the grey zone. Zone 3 is too hard to recover from easily, and too easy to produce the adaptations of real high-intensity work. It's the most common mistake.
• Wanting to "feel something". Zone 2 doesn't give you an adrenaline rush. It's measured over time.
• Eating like it's race day. The long run isn't race day. It's the quiet preparation. 5 to 20 g of carbs per hour is enough: save the gels for intensity.
• Speeding up on the climbs. The threshold doesn't care about elevation. On hills, walk if you have to.

The bottom line

The long slow run isn't sexy. It isn't fast. But it's what shifts the threshold from 56% to 88%. Without it, nothing holds.

Further reading: aerobic base training explained, why Zone 2 is controversial and metabolic health vs the aerobic glycolytic system.

Sources

• Uphill Athlete: Why is my Zone 2 so slow? (aerobic deficiency)
• Uphill Athlete: When and how to add high-intensity training, the 10 percent test
• Uphill Athlete: Zone 2 heart rate training
• Uphill Athlete: Understand the heart rate drift test
• Uphill Athlete: What is fat adaptation?
• Uphill Athlete: The science and physiology of endurance training