If you've started training for a marathon, you've probably heard someone say, "You need to do LSD." Yes, LSD is also the name of a psychedelic drug — but that's not what we're talking about here.
The problem is, most sources don't actually explain what LSD training is, why it matters, or how to put it into practice. "Just run slow and long" isn't exactly actionable advice.
This guide covers everything from the underlying physiology to real-world pace guidelines, weekly training structures, and the most common mistakes runners make — all written with beginners in mind, but useful at any level.
What Is LSD Training?
LSD stands for Long Slow Distance. At its core, it's exactly what it sounds like: running long distances (or long durations) at a low, comfortable intensity.
The concept goes back to the 1960s. Legendary New Zealand coach Arthur Lydiard famously guided his athletes to gold medals at the 1960 Rome Olympics by building massive aerobic bases through high-volume, low-intensity running. Shortly after, American running journalist Joe Henderson coined the term "LSD" in his 1969 book Long Slow Distance: The Humane Way to Train, bringing the idea to the mainstream. Decades later, it remains one of the most universally practiced training methods across both elite and recreational running communities.
LSD training has two defining principles:
- Duration or distance is longer than any other session in the week. Most runners target anywhere from 60 minutes to 3+ hours for their long run.
- Intensity is kept low — conversational pace throughout. The goal isn't speed. It's teaching your body to sustain movement over a long period without accumulating excessive fatigue.
Think of it this way: speed work raises the output of an engine that's already built. LSD training builds the fuel tank and cooling system that allow that engine to keep running for 42 kilometers.
Why Does LSD Training Matter?
A marathon isn't decided by explosive power. It's decided by your ability to sustain a consistent pace across 42.195 km — which comes down to energy efficiency and fatigue management. LSD training develops both, through several distinct physiological pathways.
Cardiac Output — Growing a Stronger Pump
Sustained aerobic training gradually enlarges the left ventricle and improves its elasticity, increasing the amount of blood the heart ejects with each beat (stroke volume). The practical result: your heart rate at any given pace drops. A more efficient heart means less cardiovascular strain at the same speed — and more headroom when the race gets tough late. This adaptation is well-documented in sports medicine as "Athlete's Heart," and it's one of the clearest long-term benefits of consistent aerobic training.
Fat Metabolism — Teaching Your Body to Use the Right Fuel
Your body shifts its primary fuel source depending on exercise intensity. At low intensities, it burns mostly fat. As intensity rises, it increasingly relies on carbohydrates (glycogen). This interplay was formalized by Brooks and Mercier in 1994 as the "crossover concept."
For marathon runners, this matters enormously. The glycogen stored in your muscles and liver is finite — roughly 400–500g, or about 1,600–2,000 kcal for a 65 kg adult. Finishing a marathon typically requires 2,500–3,500 kcal, so glycogen alone won't get you there. When you exhaust those stores, you hit the infamous "wall" — that brutal slowdown in the final miles.
Consistent LSD training increases the number and size of mitochondria in muscle cells and upregulates the enzymes responsible for fat oxidation. The result is that you burn more fat at the same pace, conserving glycogen for when you truly need it. That's the physiological foundation for holding your pace in the final 10 kilometers.
Capillary Density — Expanding the Oxygen Network
Repeated aerobic effort stimulates vascular endothelial growth factor (VEGF), triggering the formation of new capillaries and densifying existing ones throughout muscle tissue. More capillaries mean faster oxygen delivery to individual muscle fibers and faster removal of metabolic byproducts like lactate and CO₂. While high-intensity interval training also produces this adaptation, low-intensity long runs provide a sustained circulatory stimulus over a longer time window — making the two approaches complementary rather than competing.
Lactate Threshold — Going Faster Without Blowing Up
Your lactate threshold (LT) is the intensity at which lactate begins to accumulate rapidly in the blood. Aerobic training like LSD gradually pushes that threshold higher. Where you once started accumulating lactate at 6:00/km, you might eventually sustain 5:30 or 5:20/km before hitting that wall. Shifting your LT upward is one of the most direct mechanisms behind improving marathon finish times.
The 80/20 Rule — What Elite Runners Actually Do
Norwegian exercise physiologist Dr. Stephen Seiler found that world-class endurance athletes spend roughly 80% of their training time at low intensity (LSD-equivalent effort), and only 20% at high intensity. This is the "polarized training model." Most amateur runners do the opposite — they train at moderate intensity most of the time, neither easy enough for aerobic adaptation nor hard enough for peak performance gains. LSD isn't just a beginner's tool. It's what the best runners in the world rely on throughout their careers.
Mental Toughness and Race Simulation
Two to three hours on your feet does something speed workouts can't: it forces you to sit with discomfort. The boredom, the aching legs, the moments where you wonder why you signed up — that's mental training. It prepares you for the psychological low that almost always arrives after kilometer 30, and it gives you a chance to rehearse your in-race nutrition and pacing strategies under real conditions.
How Slow Is Slow Enough?
The most common question about LSD: "How slow do I actually need to go?"
The simplest rule: you should be able to hold a full conversation while running — not gasping out single words, but actually talking in complete sentences. This "talk test" isn't just intuitive; it's a validated marker of low-intensity aerobic effort in sports science research.
More specifically:
- You can speak in full sentences, not just short words
- Your breathing is elevated but controlled — you can manage it through your nose and mouth
- After finishing, you're tired but not wrecked — normal daily activity is possible a few hours later
If you prefer heart rate data, aim for 65–75% of your maximum heart rate during LSD. For someone with a max HR of 180 bpm, that's roughly 117–135 bpm. If you don't know your max HR precisely, the "220 minus age" formula gives a rough estimate — but it can be off by ±10–15 bpm, so treat it as a starting point, not gospel.
One trap many GPS-watch users fall into: assuming "slower than usual = LSD pace." If your easy pace is normally 6:00/km and you run at 6:30/km, that feels slow — but on a hot, humid day, or after poor sleep, your heart rate might still be running high. Heat alone can push HR up by 10–20 bpm at the same pace. Use pace as a reference, but trust your heart rate and breathing more.
If you're new to running slow, you'll probably feel like it's not doing anything. That instinct is wrong. The mitochondrial growth, fat-burning adaptations, and cardiac changes that LSD produces require a sustained, low-intensity stimulus over time. Running slower than you think you should is the whole point. Trust the process.
A Weekly Plan for Beginners
For runners training 3–4 days per week, the standard structure is shorter easy runs or base work on weekdays, with one LSD session on the weekend.
When starting out, it's better to measure your LSD by time rather than distance. Distance varies based on pace, which means the actual training load shifts depending on who's running and how fast. Time keeps things consistent. Also, for beginners, connective tissue — tendons, ligaments, bone — adapts much more slowly than muscle. A time-based, gradual approach helps prevent overuse injuries that come from loading the body too quickly.
| Week | LSD Target | Focus |
|---|---|---|
| Week 1 | 60 min | Get a feel for conversational pace |
| Week 2 | 70 min | Monitor heart rate to check intensity |
| Week 3 | 80 min | Start practicing in-run nutrition timing |
| Week 4 | 60 min | Recovery week — backing off is how you grow |
| Week 5 | 90 min | Focus on holding pace in the second half |
| Week 6 | 100 min | Practice full race-day nutrition strategy |