If you’ve been around marathon training circles long enough, you’ve probably heard about the so-called “3-hour rule.” It’s the idea that long runs should be capped at three hours to maximize benefits while minimizing risks. Some coaches swear by it. Others question it. And then there are those who claim it’s based on outdated rat studies from the 1960s and 1980s.
So, where did this rule actually come from, and does it still hold up today? Let’s break it down.
The Myth: Did the 3-Hour Rule Come From Rat Studies?
A common argument against the 3-hour rule is that it’s rooted in old research on rats, specifically studies by Holloszy (1967) and Dudley (1982). These studies looked at mitochondrial adaptations and found that endurance training at certain intensities led to improvements in mitochondrial density. While this research was foundational for understanding aerobic adaptations, it wasn’t a direct prescription for marathon training.
The problem? Rats aren’t marathoners. They don’t have the same biomechanical stressors, fuel needs, or mental barriers that human runners do. Taking conclusions from animal studies and applying them as hard rules for training is an oversimplification. The 3-hour guideline didn’t come from these studies—it evolved from observations of human training, injury rates, and performance outcomes.
What the 3-Hour Rule Actually Comes From
The real foundation of the 3-hour rule is based on three key factors:
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Diminishing aerobic returns – Research shows that aerobic benefits continue beyond 90-120 minutes but at a slower rate. It’s not that they stop, just that the returns decrease over time. That said, long runs are about more than just aerobic adaptations. They also build fatigue resistance, neuromuscular endurance, and mental toughness.
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The 32K (20-mile) standard – There’s a well-established correlation between runners who can complete long runs of around 32K and their marathon performance. Many elites cover this in under two hours, but for runners aiming for a 3:30-4:30 marathon, this can take closer to three hours. The guideline exists to help runners avoid excessive fatigue and long recovery times.
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Muscle glycogen depletion – The idea that glycogen stores empty after 2-2.5 hours is true to an extent, but it’s not the full story. Taking in carbohydrates during a long run delays depletion and helps maintain performance. So while glycogen depletion used to be a major concern, modern fueling strategies change the equation.
Why a Strict 3-Hour Limit Doesn’t Work for Everyone
The biggest issue with using the 3-hour rule as a hard stop is that it assumes all runners adapt the same way. The reality is that some athletes benefit from runs that go beyond three hours, while others might be better off keeping them shorter. The key is finding the right balance between getting the training stimulus you need and recovering well enough to be consistent week after week.
Here’s where I see runners struggle:
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Relying on outdated science – Some coaches cling to old research without considering newer findings on fueling, biomechanics, and recovery. If your training advice is based on decades-old studies on rats, it might be time for an update.
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Overtraining or undertraining – A rigid 3-hour cap might limit some runners who need more time on their feet. On the flip side, pushing long runs too far can lead to excessive fatigue, increasing injury risk and making it harder to hit quality sessions later in the week.
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Ignoring the mental side of training – Long runs aren’t just about physiology. They’re also about mental toughness and race-day preparation. If your longest run is only 2.5 hours but your marathon will take you 4+, you might struggle late in the race simply because you’re not used to being on your feet that long.
So, What’s the Right Approach?
Like most things in training, the answer isn’t one-size-fits-all. The three-hour guideline is useful, but it should be adapted to fit your specific needs. If you recover well and are building toward a longer race effort, extending your long run beyond three hours can be beneficial. If you’re finding yourself overly fatigued, it might be a sign to scale back.
This is why individualized coaching matters. Your training should be tailored to your experience, goals, and how your body responds—not a blanket rule that may or may not be right for you.
If you’re looking for a smarter, research-backed approach to marathon training that’s actually built for you, let’s work together. Whether you’re training for your first marathon or chasing a PR, I can help you train efficiently, fuel properly, and build the endurance you need without the guesswork.
Let’s talk about your training. Reach out, and let’s get started.