By Ryne Anderson, CTS Ultrarunning Pro Coach
Is Zone 2 the endurance hero it’s cooked up to be? Yes, and no. Research into intensity distributions among elite and everyday athletes show that the majority of training volume (about 80% for amateurs and up to 90+% for elites) should be in Zones 1 and 2. This relatively easy and sustainable intensity is essential for continued progression, improvement, and longevity as an athlete. However, we also know that high-intensity training improves multiple physiological markers that correlate with higher aerobic capacity. Intensities of Zone 3 and above may constitute 20% of your training volume, or less, but it can make a tremendous difference in your performance, finish times, and the fun you have during events. All zones matter. All training zones deserve your attention, and athletes who avoid going hard for fear of disrupting the gains from low-intensity training are leaving huge performance opportunities on the table.
Why go hard in training when ultramarathon race pace is moderate?
Over the course of an ultramarathon, runners sustain an average intensity of about 60% of VO2 max. This includes harder periods at or above lactate threshold pace (70-80% of VO2 max) and easier periods at a walking pace (20-30% of VO2 max). Most ultrarunners spend very little to no time at intensities approaching VO2 max (i.e., maximum aerobic capacity) during an event. And yet, most training methodologies targeted to ultramarathon runners incorporate intervals at and above lactate threshold (e.g., Zone 4, TempoRun, long hill repeats, etc.) and short, hard intervals focused on increasing VO2 max.
Capacity is the reason to train harder than you’re going to run during your events. Focused work at higher intensity zones creates the physiological adaptations that raise your maximum capacity for work, which also raises the amount of work you can perform at easy and moderate intensities. In other words, your Zone 2 pace gets faster from the adaptations created from working out in Zone 5.
To learn how this works, let’s start by defining training zone systems.
Defining the 5 Zone System
I’m going to start with the 5-zone training intensity model CTS Coaches use to guide training intensity, with each zone anchored to key physiological markers like aerobic and lactate thresholds. There are other zone systems that try to simplify the process (e.g., a 3-zone system that’s essentially easy/medium/hard) or make training even more precise (e.g., a 7-zone system).
Zone 1 represents active recovery, a very easy effort with minimal metabolic strain.
Zone 2 is steady aerobic work below the first lactate threshold (LT1), where fat oxidation is maximized and lactate remains at baseline.
Zone 3 spans the space between LT1 and the second lactate threshold (LT2), often called Steady State. It’s challenging enough to accumulate fatigue, but below the redline.
Zone 4 targets LT2, where lactate begins to accumulate rapidly; this is the top of steady-state aerobic metabolism.
Zone 5 is above lactate threshold. VO2 max efforts that stress the cardiovascular and neuromuscular systems and rely heavily on anaerobic metabolism.
Why do ultrarunners love Zone 2?
Besides the well-established physiological benefits of a large volume of Zone 2 training (i.e., improved aerobic capacity, improved mitochondrial density and function, improved fat oxidation, etc.), there are some practical and behavioral reasons ultrarunners love Zone 2:
- It’s not that hard: Zone 2 is a comfortable, all-day pace that is pretty enjoyable. Harder efforts are less comfortable and interval workouts require more focus to hit target intensity ranges and effort durations.
- Recovery is relatively quick: Experienced runners can complete multiple Zone 2 runs per week because the workload of each session is low enough that they can recover within 24-48 hours and go for another run. Harder intervals are more taxing and require longer recovery periods.
- Acute injury risk is lower: Higher intensity training can increase injury risks because of higher forces and velocities, as well as higher acute workloads that must be balanced with adequate recovery. Injury risks from a higher volume of low-intensity training are more from overuse injuries or a chronic imbalance between workload and recovery.
The best thing about Zone 2 running is that one of the most beneficial training intensities aligns with the type of running most athletes enjoy the most. A runner who only completes Zone 2 training will most likely stay healthy and relatively injury free, which means they will be able to train consistently. I will always preach that consistent training is the number one driver of preparation and development. So why take the risks of introducing higher intensity training and jeopardizing that consistency?
Why is it important to train at intensities greater than Zone 2?
Although Zone 2 is sustainable and the body can handle it well, everyone has a limit in terms of available training time and there are limits to the physiological adaptations a runner can expect from Zone 2 training. When you reach the highest level of volume you can sustain at Zone 2 without causing injury or overtraining, you need to explore additional ways of creating a training stimulus. Otherwise, fitness plateaus.
This is where high-intensity training comes into play. Intensity and speeds above Zone 2 come at a greater physiological cost but deliver great rewards. A higher VO2 max and pace at lactate threshold correlate with stronger overall fitness and performance. Among many other studies, this was demonstrated in this study by CTS coach and researcher Frederic Sabater Pastor, who found that VO2 max and percentage of VO2 max at Lactate Threshold were significant predictors of performance in a 166km race.
How Higher Intensity Training Works
High intensity is defined as work at Zones 3, 4, and 5. As intensity increases, the amount of time-at-intensity required to achieve a meaningful training stress decreases. This means that individual efforts at higher intensities are shorter, and that the total weekly and monthly accumulated time-at-intensity in these zones will represent a smaller percentage of your overall training time. The duration of high-intensity intervals is dictated by how long you can sustain the efforts. The volume of work at these high intensities is then dictated by how many times you can repeat those efforts. Viewed this way, it’s understandable that high-intensity training constitutes a low percentage of your overall training time because you physically can’t complete a lot of it.
In the table below, individual Zone 3 intervals are described as 20- to 60-minute efforts. More experienced athletes may be able to maintain a Zone 3 effort for 60 minutes. If you can run at Zone 3 for three hours, then it’s time to retest and reset your training zones. This would be a sign of progress, because it means your old Zone 3 pace is now your new Zone 2 pace! The same goes for other intensity zones. Zone 5 can only be sustained for up to about five minutes, maybe a little longer for elites. Once your Zone 5 pace/effort feels easier and can be sustained longer, it’s a sign you’ve made progress and it’s time to reset your ranges.
| Interval Name | Zone | RPE (1-10 scale) | Typical Interval Time | Total Time-at-Intensity | Work:Rest |
| RecoveryRun | 1 | 4-5 | NA | 20-60 min | NA |
| EnduranceRun | 2 | 5-6 | NA | 30 min to 6+ hr | NA |
| SteadyStateRun | 3 | 7 | 20-60 minutes | 30 min to 2 hr | 5-8:1 |
| TempoRun | 4 | 7-8 | 8-20 minutes | 30-60 min | 2:1 |
| RunningIntervals | 5 | 9-10 | 2-5 minutes | 12-24 min | 1:1 |
How to schedule recovery during high intensity training
High intensity training is somewhat self-limiting, in that your fitness level limits how long you can maintain the target intensity. Where athletes get in trouble is failing to schedule enough recovery time between high-intensity workouts. For most athletes, one day of interval training per week is safe and sufficient. Some athletes can handle two interval workout days per week, particularly if the intervals are Zone 3 or 4. A good rule of thumb is to space interval workouts out so you have at least 48 hours between hard efforts, even if you have a recovery activity or complete an endurance run within that time span.
How to implement high intensity training
The simplest way to implement high-intensity training is to work backwards from your goal event and plan out your high-intensity phases as far away from your event as possible.
Let’s use Western States as a practical example in terms of designing training phases. Western States always takes place on the last Saturday in June. January through March would constitute cycling through phases of Zones 3, 4, and 5. The simplest format would be to perform a 3 week block of work, followed by a recovery week. Perform two workouts per week at the intensity you are targeting. January would target Zone 5, February would focus on Zone 4, and March would hit Zone 3. April through June would focus on higher volume phases that have a heavier emphasis on Zone 2 while still including work at Zone 3 to maintain quality.
This example is an oversimplification of a long-range plan, but it is meant to illustrate the principle of going from least to most specific physiology in the build towards your goal race. Each athlete is unique in terms of their injury history, training history, strengths, weaknesses, time budget to train, race schedule for the year, and goals of their race. These considerations factor into decisions such as how long and frequently to spend at a specific intensity, as well as how to organize the flow and total time of intensity. But the point always remains to go from least specific physiology to most specific physiology in the build towards your goal race.
More than Zone 2 leads to continued improvement
Basic training theory is rooted in overload, rest, and adaptation. The best thing about Zone 2 is that you can accumulate enough time at this training intensity to stimulate great adaptations, and it’s so hard that you risk acute injuries or must be especially careful about scheduling recovery. However, if you always train at the same intensity and load, your fitness will plateau and you won’t increase your overall capacity for aerobic work. Higher-intensity interval training is crucial for elevating your fitness and performance beyond what you can accomplish through low- to moderate-intensity running.
References:
Gallo G, Mateo-March M, Gotti D, Faelli E, Ruggeri P, Codella R, Filipas L. How do world class top 5 Giro d’Italia finishers train? A qualitative multiple case study. Scand J Med Sci Sports. 2022 Dec;32(12):1738-1746.
Gallo G, Mateo-March M, Gotti D, Maunder E, Codella R, Ruggeri P, Faelli E, Filipas L. The Weekly Periodization of Top 5 Tour de France General Classification Finishers: A Multiple Case Study. Int J Sports Physiol Perform. 2023 Sep 14;18(11):1313-1320.
Sandbakk Ø, Tønnessen E, Sandbakk SB, Losnegard T, Seiler S, Haugen T. Best-Practice Training Characteristics Within Olympic Endurance Sports as Described by Norwegian World-Class Coaches. Sports Med Open. 2025 Apr 25;11(1):45.
Sabater-Pastor, F., Tomazin, K., Millet, G. P., Verney, J., Féasson, L., & Millet, G. Y. (2023). VO2max and Velocity at VO2max Play a Role in Ultradistance Trail-Running Performance. International Journal of Sports Physiology and Performance, 18(3), 300-305. Retrieved Oct 24, 2025, from https://doi.org/10.1123/ijspp.2022-0275
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