Zone Training Explained: Why Those 'In-Between' Miles Still Matter

Jan 5, 2025

Introduction

Zone training has become a cornerstone of structured running programs. By dividing your effort into distinct bands—commonly labeled Zone 1 through Zone X—you can more precisely target the physiological adaptations you’re seeking, from developing aerobic endurance to sharpening race-pace efficiency. Still, many runners worry about hitting a so-called “gray area” or “junk” territory between zones, fearing they’ll water down their training.

This article explores how zones are defined, why each one matters, and why the paces that fall between “ideal” targets are often more beneficial than we think. Spoiler alert: they’re rarely junk. Instead, they contribute important adaptations that can sustain performance, prevent burnout, and refine your body’s ability to handle a range of intensities.

1. What Is Zone Training?

At its core, zone training is simply a method of structuring your runs based on effort or physiological markers. By slicing up your effort level into distinct “zones,” you can systematically work on different aspects of your fitness.

Origins: Early running coaches and exercise physiologists noticed that training at different intensities could produce unique benefits. Over time, they codified these intensities into zones.
Goal: Rather than running every workout at a random easy or hard pace, zone training aims to match your run to a specific adaptation. For example, you might run in an easy “Zone 2” to develop your aerobic base or push up into a threshold zone to improve your lactate clearance.

2. Common Ways to Define Your Zones

The exact number of zones can differ based on the method you follow—some models use three zones, while others go up to seven—but the principles generally remain consistent.

Heart Rate–Based
- Zones are often pegged to percentages of your maximum heart rate (Max HR).
- Example: Zone 2 might be roughly 60–70% of your Max HR.
Pace–Based
- Runners set target paces for each zone based on recent race or time-trial performances.
- Example: Zone 4 might be around your 10K race pace.
Lactate or Ventilatory Threshold–Based
- Zones hinge on physiological breakpoints (e.g., lactate threshold).
- Lab testing or field tests can identify these breakpoints precisely.
Power–Based (Newer Trend)
- Technology like Stryd foot pods helps define zones by running power (watts).
- Similar concept: different power levels map to different effort zones.

Despite variations in labeling, every system aims to pinpoint how your body changes as you push harder.

3. Physiological Overlaps: Why Zones Aren’t All-or-Nothing

The human body doesn’t magically switch from one energy system to another the instant you cross a threshold pace. Instead, all energy systems are on a sliding scale, with certain systems ramping up and others scaling down as intensity shifts.

Dimmer Switch Analogy: You can think of your aerobic and anaerobic systems like two lights in a room. They’re both on at various levels, adjusting based on how much power you need.
Transitions Are Gradual: Zone boundaries help guide training, but they aren’t walls. Running just above or just below your threshold can still overlap many of the same training effects.

Because of this continuum, many coaches believe no pace is inherently “bad” as long as you connect it to a purpose within your plan.

4. Why “Between” Miles Get a Bad Rap

So if every intensity has a potential benefit, why does the term “junk miles” or “gray area” appear at all?

Misinterpretation of Goals: If your aim is to improve threshold with a strict block of zone-specific training, you might see paces just outside that sweet spot as suboptimal.
Over-Training Risk: Sometimes runners hover in a moderately hard intensity—often called “Zone 3” in certain 5-zone models—too often, tiring themselves without gaining the full benefit of either a truly easy run or a purposefully high-intensity session.
Historical Confusion: Early zone-based guidelines, including some from famous running texts, suggested limited time in certain pace ranges because they didn’t align perfectly with specific adaptation markers (like VO₂ max). Later research, however, showed that these paces still offer valuable benefits.

5. Why Every Zone Matters

Let’s take a closer look at how each major zone (assuming a 5-zone model) ties into performance, including mild intensities that might otherwise be dismissed.

Zone 1 (Active Recovery/Easy)

Effort Level: Very comfortable; you can hold a conversation easily.
Physiological Focus:
- Blood flow promotion, assisting recovery.
- Gentle aerobic stimulation.
- Lower stress on muscles and joints.
Typical Benefits:
1. Minimizing injury by ensuring low-impact mileage.
2. Improving fatigue resistance over time, especially for newer runners.
3. Boosting mental readiness for harder workouts.

Zone 2 (Endurance/Base Builds)

Effort Level: Slightly elevated breathing but still quite manageable.
Physiological Focus:
- Expansion of aerobic capacity.
- Enhancement of fat-burning efficiency.
- Ongoing musculoskeletal adaptations.
Typical Benefits:
1. Solidifies your aerobic base—often the foundation for marathon training.
2. Allows for accumulation of volume without overly taxing your system.
3. Helps build capillary density, improving oxygen delivery.

Zone 3 (Steady/Moderate)

Effort Level: A moderate burn; conversation is possible but strained.
Physiological Focus:
- Overlaps with many adaptations (aerobic, muscular endurance).
- Gets you comfortable operating just below threshold.
- Can improve “bridge” from easy running to harder intensities.
Typical Benefits:
1. Strengthens cardiovascular system without the toll of threshold workouts.
2. Familiarises you with controlled discomfort, beneficial for longer races.
3. Maintains aerobic capacity gains while nudging up intensity.

Zone 4 (Threshold/Tempo)

Effort Level: Sustained discomfort; you can say short phrases, but it’s a challenge.
Physiological Focus:
- Increases lactate clearance and tolerance.
- Improves running economy at paces close to race intensity (10K to half-marathon, typically).
- Builds mental resilience for racing.
Typical Benefits:
1. Widely recognised for helping boost performance in mid-distance races and half marathons.
2. Teaches your body to deal with and recycle lactate.
3. Often a staple in training plans for advanced runners.

Zone 5+ (VO₂ Max & Beyond)

Effort Level: Very intense; conversation is basically impossible.
Physiological Focus:
- Maximises oxygen uptake.
- Trains fast-twitch fibers for speed and power.
- Sharpens race-specific fitness for shorter events.
Typical Benefits:
1. Significant cardiovascular stress leading to VO₂ max improvements.
2. Develops speed and finishing kick for track or road races.
3. Helps break through performance plateaus.

Key Takeaway: Zones 1–5 create a tapestry of training intensities that all serve a purpose.

6. Debunking the “Junk Miles” Label

“Junk miles” is sometimes a blanket term for running that isn’t at a specifically targeted pace. But out-of-zone or in-between intensities still carry physiological weight.

Structural and Aerobic Adaptations

Bone Density & Tendon Strength: Repetitive loading at moderate intensities—arguably between Zones 2 and 3—helps strengthen bones, tendons, and connective tissues incrementally without the high stress of speed work.
Glycogen Utilization: The body learns to use both carbohydrates and fats more effectively at these transitional paces, expanding your metabolic flexibility.

Mental and Recovery Benefits

Stress Relief: Running at a pace that feels pleasantly challenging yet not draining can be a psychological lift.
Recovery Day Variety: If you’re feeling good, “in-between” paces can keep you engaged without digging into your reserves, preventing boredom and keeping sessions fresh.

The Importance of Variety

Avoiding Monotony: Overemphasizing just one or two zones can lead to stagnation or injury.
Balancing Different Fiber Types: Running at slightly faster or slower speeds than intended can engage and train different muscle fibers—some slower-twitch, some more intermediate.
Long-Term Consistency: Sustainable programs usually include a range of intensities to maintain motivation and limit overuse injuries.

Remember: Building a robust performance engine demands exposure to a wide variety of intensities.

7. Strategies to Blend Zones Without Overdoing It

Mastering zone training means blending intensities wisely while ensuring each run has a clear purpose. Below are a few approaches:

Polarised Approach (Approx. 80/20 Split)
- Run most mileage in Zones 1–2.
- Perform a few weekly sessions at Zone 4–5.
- Bottom Line: You minimise moderate-intensity work, but not entirely.
Pyramidal Approach
- Heavier proportion of easy runs (Zone 1–2).
- Moderate volume of steady/tempo (Zone 3–4).
- A small fraction of VO₂ max or speed reps (Zone 5).
- Bottom Line: Volume at moderate intensities can lift overall aerobic capacity.
Threshold-Focused Approach
- Large chunk of training near threshold (Zone 4).
- Still includes easy runs (Zone 1–2) for volume.
- Bottom Line: Great for advanced runners refining half-marathon or 10K performance.
Hybrid or Rotational Model
- Periodise your training: focus on a threshold-heavy block, then shift to polarised leading up to race season, for instance.
- Bottom Line: You adapt your zone balance throughout the year to match different training goals.

8. Signs You’re Misusing (or Overusing) Your Zones

Even well-intentioned runners can slip into suboptimal use of zones. Here’s how to spot potential issues:

You Always End Up in the Same Pace Range: If every run is “moderately hard,” you might be drifting into a single zone more than planned.
Chronic Fatigue or Plateau: Overdoing middle or high-intensity zones without enough true recovery can lead to stagnation or burnout.
No Variation in Performance: If your easy runs are too fast or your hard runs are too tentative, you aren’t truly hitting the intended stimuli.
Elevated Resting Heart Rate: Could be a sign you’re under-recovering.

Fix: Add more genuine Zone 1 or 2 runs for recovery, then keep your high-intensity days focused and intentional.

9. How to Progress Within and Between Zones

Progression doesn’t always mean upping the speed; it can also mean fine-tuning volume, duration, and density within each zone.

Increase Volume First: If you’re comfortable running 45 minutes in Zone 2, try 60 minutes at the same heart rate the next time.
Add Intervals or Over-Unders: Once you master a zone duration, include short surges into a slightly higher zone, then return to your target zone.
Shorten Rest Periods: When building threshold or VO₂ max intervals, reducing rest times can challenge your endurance without significantly changing pace.
Test Yourself Periodically: Time-trial a 5K or 10K to ensure your zone definitions remain accurate as fitness improves.

10. Practical Tips for Adding “In-Between” Workouts

These extra micro-adjustments let you tap into beneficial intensities without overshadowing your main training:

Progressive Long Run
- Start in Zone 2, finish in Zone 3.
- Benefits: Gentle shift from base-building to moderate effort.
Fartlek Sessions
- Unstructured intervals varying from Zone 3 to Zone 4.
- Benefits: Teaches pace control and transitions between intensities.
Steady-State Finishers
- After an easy run, add 10–15 minutes of strong but not all-out pace (upper Zone 2 or low Zone 3).
- Benefits: Invigorates leg turnover while preserving the essence of an easy day.
Back-to-Back Zone Variance
- Day 1: Strict threshold intervals (Zone 4).
- Day 2: Moderately paced run in Zone 3.
- Benefits: The second day can reinforce some threshold adaptation without the stress of another peak session.
Mixed-Intensity Long Repeats
- Example: 2×20 minutes with 5 minutes at upper Zone 2, 5 minutes at mid Zone 3, repeating.
- Benefits: Systematically trains multiple intensities in one session.

Tip: Always plan these thoughtfully around your truly hard days and rest days to avoid compounding fatigue.

11. Final Thoughts

Zone training isn’t about boxing yourself into narrow slices of pace to the point of rigidity. Instead, it’s a framework that shows how each intensity can cultivate specific adaptations. While you’ll often hear warnings about “gray areas” or “junk” miles, real-world experience and a growing body of research reveal that most intensities—even those in-between paces—have something to offer.

Balance: Avoid spending all your time in the same zone—especially if it’s moderately hard—because that’s where recovery can become an issue.
Purpose: Every run should have a rationale, whether it’s an easy recovery, moderate base-building, tempo training, or VO₂ max intervals.
Personalisation: Not every runner’s zones align perfectly on a chart. Individual factors like muscle fiber composition, running background, and personal goals matter.

Ultimately, the best zone-based plans recognise that the body adapts through a wide spectrum of intensities over time. There’s often a place for in-between paces—they can smooth the gradient of intensity in your training and bolster both physical and mental resilience.

12. Disclaimer

This article is intended for educational purposes only and should not replace professional medical advice. Always consult a qualified coach or healthcare provider before making substantial changes to your training routine, especially if you have any underlying health conditions.

By embracing and intelligently blending all zones—yes, even those in-between efforts—you’ll be well on your way to a more robust, adaptable, and enjoyable running practice.