What to Know About Gray Zone Training and How It Can Help—or Harm—Your Running
A well-thought-out training plan, particularly for a marathon, challenges runners to complete a variety of workouts designed to strengthen the body in different ways. Each run, then, serves a particular purpose. Easy days should remain easy; if you run them too hard, your body can’t recover for the next workout. On the flip side, hard days—interval workouts, long runs (even if they’re at an easy pace), hills—are meant to be hard, to help your body adapt and improve fitness.
But then there’s the dreaded (or perhaps, misunderstood!) gray zone. Even its name implies a murkiness to this area of training.
“People spend a lot of time here, hovering in the gray zone instead of prioritizing harder efforts and easier efforts,” says Erika Kemp, a professional marathoner with Brooks Running, who most recently coached Cynthia Erivo to a 3:21 marathon.
When used correctly, however, the gray zone—also known as zone 3 in a typical five-zone model—can improve your training and performance.
So how do you use the gray zone to your advantage instead of letting it stall your progress? Here’s what to know.
What Is the Gray Zone?
The gray zone is a tricky zone because there isn’t really a test for it to find your pace, like lactate threshold or a VO₂ max, says Mike McMillen, head coach and founder of North Carolina-based Flow Motion Running. For most runners, he says, the gray zone tends to be the top end of easy running, drifting into a steady-state effort—a little slower than marathon pace for most people.
“Technically, the gray zone is around the first aerobic threshold… Heart rate, which varies from athlete to athlete, is somewhere around 75 to 80 percent of the max heart rate,” McMillen says. “The effort isn’t quite easy but it’s not hard. It’s a moderate intensity.”
But this isn’t all that helpful for runners who don’t know their precise lactate threshold or maximum heart rate, McMillen says. Instead, runners should really focus on feel to get a sense of this effort and pace.
“Most people will tell you the gray zone feels like a 5, maybe a 6 on occasion, on a scale of 10, where you can do it and it feels good and it’s something you can do for a long period of time without really struggling,” he says. Another way to think about it: You can tell you’re working more than a recovery run.
If we go to the talk test, McMillen says runners will be able to chat pretty comfortably in the gray zone, maybe having to stop at some point to catch their breaths if the pace quickened when the conversation got “juicy.” This is why, he says, the gray zone feels so good—you can do it comfortably, but once you’ve finished you can tell you did some work.
Running apps like Strava show a breakdown of your zone efforts, including the gray areas of zone 3, but McMillen cautions against the reliance on these analyses, which are based on a device like a watch, chest, or armband for heart rate. If the device isn’t accurate (the gold standard is a chest or armband) or you don’t know your max heart rate, the interpretation of the data will be inaccurate.
However, related to wearables, Kemp says that the color-coding system on rings, watches, and other trackers can be helpful. Although she doesn’t think it’s necessary to check on the fitness stats every day, peeking at your runs—and the effort level at which you ran them—can provide insight into how hard (or not) you’re running and whether anything is out of balance.
The Benefits of This Training Zone
When dosed appropriately, training in the gray zone has real benefits, particularly for building an aerobic base, Kemp says—and particularly for the marathon distance.
“From a psychological standpoint, you’re running at an intensity that’s comfortable but also gives yourself exposure to mild discomfort, knowing that 26.2 miles is at a similar effort,” McMillen says.
Physiologically, the gray zone prepares your body to burn a combination of fat and carbohydrates, which is what you’ll be doing during a marathon. But the gray zone tends to burn glycogen (the stored form of carbs) more quickly than easier efforts, à la zone 2, which makes the runs more tiring and recovery time longer.
During a marathon block, this gray zone training also gets your body used to running close to your race pace without exhausting your body by running too many miles at that pace, McMillen says. (That’s called the specificity of training: exposing your body to what race day will feel like so it can adapt accordingly.)
The gray zone won’t automatically harm training for a 5K or 10K; you’ll just gain the most advantages for the marathon and some for the half marathon.
The Downsides
Like with any training tool, too much of a good thing can turn bad—and that includes the gray zone. “When you’re always training in the gray zone, you’re stuck,” McMillen says. That’s because the gray zone is not sustainable for long-term development.
“It adds stress at a time when we’re trying to reduce it, which compromises the recovery process,” he says.
Kemp calls training weeks that are almost entirely in the gray zone “one note.” In other words, a sign that you’re spending too much time here is that your paces are all generally the same, somewhere between hard and easy. This might negatively affect your performance because you’re not necessarily adapting to the demands of tough interval sessions, nor are you fully recovering with easy runs.
So if you’re hitting a plateau in performance or you’re struggling in speed workouts, it’s time to reassess your paces and efforts. “If we’re limited in how fast we can run on harder days, it might be because we’re carrying a lot of fatigue from the gray zone,” McMillen says.
This is why Kemp likes to stay in touch with speedwork during marathon training, particularly with strides or intervals at a pace slightly faster than half marathon. “It gives a sense of how your legs and body are feeling,” she says. “If you’re feeling heavy and not recovering, obviously something is a little off. When you’re solely in the gray zone you may not realize that.”
Struggling on workouts every now and then is normal; it’s when you’re consistently having a hard time that suggests an underlying concern—like not working easy enough on easy days.
Finally, although overtraining in the gray zone doesn’t directly cause overuse injuries, it creates the conditions (like underrecovery) that make those injuries more likely, McMillen says.
How to Train in the Gray Zone to Actually Gain the Advantages
With the popularity of polarized training—easy days are easy, hard days are hard—the gray zone has gotten a bad rap, McMillen says. “People steer away from it completely. But if you use it in the right way, it’s extremely beneficial,” he explains.
Just how often you should train in the gray zone or zone 3 really depends on the runner, McMillen says, noting he counts it as a quality day because of its physiological cost.
For beginners, he’s not prescribing these workouts, but he knows the gray zone efforts can sneak up on runners so he’ll factor that in when building a plan. For more experienced athletes, McMillen will prescribe it, thoughtfully, as part of a run once every one to two weeks. Or, he says, roughly less than 15 percent of their weekly volume.
Gray zone training is a tool, just like your easy runs are a tool to build endurance, and your hard runs are a tool to raise your VO₂ max and lactate threshold. Gray zone workouts can improve fatigue resistance, McMillen says, or the ability to keep pushing at the end of a long race when you feel tired because you’re asking your body to hold a pace close to race pace for a long time.
Gray Zone Workouts
Long Run With Gray Zone Pacing
During a long run, focus on easy pace for the majority of the workout. Then accelerate to your gray zone effort for the last several miles (aiming for that 5 to 6 RPE). For example, in a 14-mile run, the first eight miles should be easy with the last six miles in the gray zone.
“That little bit of discomfort at the end is intentional and will prepare you for race day,” McMillen says.
Progression Run
A progression run allows you to experience different intensities, which teaches you how your body adjusts to picking up the pace during a race. Try a midweek eight-mile progression run: Start easy, then increase speed to your gray zone effort, then increase to marathon pace, and finish slightly faster than marathon pace.
Mile Repeats With Gray Zone Recovery
Run 5 x 1-mile repeats at marathon pace and recover for three minutes at your gray zone pace (again, just above your easy pace). This will not only prime your body to run on tired legs, but it will teach you how to understand what the gray zone feels like.
Heather is the former food and nutrition editor for Runner’s World, the author of The Runner’s World Vegetarian Cookbook, and a nine-time marathoner with a best of 3:23. She’s also proud of her 19:40 5K and 5:33 mile. Heather is an RRCA certified run coach.
