Calculating Long-Run Pace: The Science Behind 90 Seconds Slower Than Marathon Goal Pace

Introduction

“How fast should a long run be?” is one of the most frequently asked questions in marathon training. Many runners intuitively believe that long runs should be as close to race pace as possible to effectively practice race feel. However, sports science research and the training systems of the world’s top marathoners all point to the same conclusion: the pace of weekly long runs should be approximately 60–90 seconds/km slower than your target marathon pace. This recommendation is not arbitrary—it is grounded in solid metabolic physiology.

Why Shouldn’t You Run Long Runs at Race Pace?

The Metabolic Fuel Crossover Point

During aerobic exercise, the body burns both fat and carbohydrates (glycogen) simultaneously. As exercise intensity increases, the proportion of carbohydrate utilization rises rapidly:

A full marathon is typically run at 75–82% of maximum heart rate. At this intensity, carbohydrates (glycogen) are depleted rapidly, and the body’s muscle glycogen stores can only sustain about 90–120 minutes of high-intensity exercise—this is the physiological root of “Hitting the Wall.”

If every long run is done at race pace, glycogen depletion is fast, lactate accumulation is high, and recovery takes longer. Not only does this make it difficult to fit adequate training volume into a week, but over time it also increases the risk of overtraining.

The Metabolic Logic Behind the 90-Second Slowdown

When pace drops to 75–90 seconds/km slower than marathon goal pace, exercise intensity typically falls to 65–72% of maximum heart rate. At this level:

• Fat metabolism increases to 55–65%, providing the strongest stimulus to fat oxidation enzymes
• Glycogen depletion rate drops by approximately 30–40%, allowing runners to complete 28–32 km long runs without hitting the wall
• Lactate levels remain low and steady (typically below 2 mmol/L), significantly shortening recovery time (24–36 hours vs. 48–72 hours for high-intensity efforts)

Conclusion: Running 90 seconds slower is not slacking off—it’s what allows the long run to deliver maximum benefit in the correct metabolic zone.

Personal Long Run Pace Calculation Table

Below are recommended long run paces corresponding to common marathon goal times in Taiwan:

Real-World Factors Affecting Long Run Pace

Taiwan’s Climate Factors

Taiwan’s summers are hot and humid, with the perceived temperature often 5–10 degrees higher than the actual air temperature. Heart rate at the same pace can be 5–15 bpm higher than in cooler months. Here are pace adjustment recommendations by temperature:

Terrain Factors

• Uphill: Heart rate rises, so you need to actively slow down (even walk) to maintain your target heart rate
• Downhill: Although heart rate may drop, muscle impact increases, so pace should not be too fast either

Heart Rate vs. Pace: Which Is More Reliable?

In long run training, heart rate is primary, pace is secondary:

• Temperature, humidity, altitude, and fatigue levels all affect the relationship between heart rate and pace
• Keeping heart rate within the target zone under any conditions ensures the long run is performed at the correct metabolic intensity
• It is recommended to use a heart rate monitor (with a chest strap sensor) for long runs—chest straps are far more accurate than optical wrist-based sensors

Practical Recommendations

1. Set a heart rate target first, then observe the pace: Set your heart rate target at 65–72% of max HR, see what pace you naturally settle into, and use that pace as a reference going forward
2. Don’t compare pace with running buddies: If you encounter faster runners on your long run, hold your own pace; the long run is a personal aerobic foundation-building project
3. Use pace bands: Print or record your target long run pace and attach it to your wrist as a reminder of your upper limit
4. Progressive long runs (optional): Advanced runners can accelerate to marathon goal pace for the final 5 km of a long run, combining aerobic base building with race pace adaptation

Conclusion

The seemingly contradictory training philosophy of “run slow to run fast” is built on rigorous metabolic physiology. Before your next Sunday long run, look up your target marathon pace, then set your long run pace 75–90 seconds slower. Let every slow kilometer accumulate aerobic foundation in the most efficient metabolic zone. Time will prove that this “slow” decision is the smartest choice in your training plan.

Related Reading

• Pacing Control for Long Runs: The Long-Term Benefits of Running Conservatively
• Marathon Long Run Training Strategies: A Scientific Approach to Building Distance
• Executing the Long Run Correctly: Pace, Distance, and Frequency
• Marathon Pace Training: Precisely Mastering Your Goal Pace