In marathon and distance training the "grey zone" and "junk mileage" are repeatedly mentioned. Many people simply understand them as the awkward range of "not slow enough, not fast enough." But is the so-called "grey zone" really completely undesirable? Are the "junk miles" that many runners avoid truly all harm and no benefit? To understand this question we need to focus on one core concept — Zone 3 heart rate. To understand the "grey zone," first grasp a basic training principle: different training days should serve different purposes. Easy runs and recovery runs are meant to accumulate aerobic time at a relatively low physiological cost, promote circulation and recovery, and preserve readiness for subsequent key sessions; tempo runs, interval sessions and long race-specific runs are tasked with improving speed endurance, cardiopulmonary capacity and race adaptability. This is also what polarized training demands: be relaxed when you should be relaxed, and be capable when you need to be. A mature runner is not only someone who can run fast, but someone who can accurately control pace, heart rate and perceived effort according to the purpose of each session. The so-called "grey zone" typically appears when this intensity distinction breaks down. For example, a workout that was supposed to be an easy or recovery run can, in practice—especially for beginners, when they're feeling good, running with a faster group, or overly focused on pace data—unconsciously accelerate, so what should have been an easy session slides toward moderate intensity. In the end, heart rate stays in Zone 3 for a long time, perceived effort is no longer easy, yet the intensity still hasn't reached that of a true tempo or high-quality session. At that point, Zone 3 heart rate becomes the so-called "grey zone." It makes the runner pay a higher fatigue cost than an easy run without delivering commensurate training benefit. After the run it can even create the illusion of a solid, satisfying workout, while actually disrupting the body's recovery rhythm and potentially degrading the quality of the next key session. So what truly needs warning is not Zone 3 heart rate itself, but those unplanned, unbounded, purposeless moderate-intensity efforts. Compared with standard easy runs, running near Zone 3 brings higher energy expenditure. An easy run should be a low-cost aerobic accumulation the body can complete relatively relaxed to support circulation, metabolism and base adaptation. Once intensity rises, glycogen depletion, cardiopulmonary stress and subjective fatigue all increase. The grey zone also increases muscular and nervous-system load. Running at Zone 3 is usually accompanied by faster pace, higher cadence, or greater mechanical output; the leg muscles must bear more contraction stress and impact than during an easy run. It doesn't have the clear stimulus targets of real speed work, yet it still leaves fatigue. This effect is most likely to show up in the next hard session. Many runners have the experience: they did an "aerobic" easy run the day before, but the next day when they try intervals or a tempo they find heavy legs, elevated heart rate and an inability to hit target paces. Therefore, scientific training isn't about running to exhaustion every day, but about letting different sessions carry distinct roles. Easy runs handle recovery and aerobic accumulation; tempo runs handle sustained output; intervals provide high-intensity stimulus; long runs develop endurance and race-specific adaptation. If easy runs frequently drift into Zone 3, the boundary between "easy" and "hard" in the program becomes blurred. The result is runners end up training every week but never truly easy, while performance gains slow. The cause isn't necessarily a lack of effort but a lack of differentiation in training. The body remains stuck in repeated moderate-intensity depletion, receiving neither adequate recovery nor sufficiently distinct high-quality stimulus.This is especially apparent for runners whose recovery time is limited — the grey zone's impact will be greater, and any extra expenditure can affect subsequent sessions. Discussing Zone 3 heart rate cannot be separated from one premise: the runner's training volume and recovery capacity. The same Zone 3 session can mean completely different things for different runners. For a low-volume runner who trains two or three times a week it may be an effective stimulus; but for someone training five or six times per week in a structured race-preparation cycle, it can become a major source of cumulative fatigue. This is also where "junk mileage" is most easily misunderstood.It is not to say that all moderate-intensity training is without value.Instead, it depends on where this intensity sits within the entire training cycle and its proportion, and whether it affects physical recovery and subsequent key workouts. For high-mileage runners, distinguishing intensity is especially important. When a runner already schedules a lot of training per week, an easy run is not just a “casual jog”—it serves to maintain the aerobic base, increase volume, promote recovery and link key workouts. At this stage, whether a runner can continue to improve often depends not on running every session all-out, but on whether the body can consistently recover under a higher training frequency. If high-mileage runners frequently run easy runs into Zone 3, training load will quietly increase. That extra fatigue may not be obvious in the short term, but over a week or a month it can accumulate. The usual outcome is: daily training looks hard and the data don’t look bad, but on the sessions that really require effort—hard workouts, long runs, or race day—the runner can’t produce the desired performance. Low-mileage or time-limited runners are different. If a runner trains only 2 to 3 times per week and has ample recovery between sessions, the body’s tolerance for moderate intensity is usually higher. In that case, spending some time in Zone 3 can provide a fairly direct cardiopulmonary stimulus, because total volume is low and recovery demands are relatively controllable. But this pattern is not suitable as a long-term reliance. As training level improves, depending on “feeling a bit tired” every session yields diminishing returns. Especially when a runner increases mileage or enters half- or full-marathon preparation, the Zone 3 work that was once tolerable can become a recovery burden. Therefore, the pros and cons of Zone 3 heart rate are not absolute. What truly determines its value is not the heart-rate zone itself, but whether it matches overall training volume, recovery capacity and training purpose. For marathon and half-marathon training, compared with short intervals or high-intensity sprints, Zone 3 heart rate does not emphasize maximal output but maintaining stable running for a relatively long time in a controllable state. This intensity is close to the condition many recreational runners actually face in races: breathing is no longer completely easy, the legs begin to feel the load, but with pace control you can still keep pushing. For example, in marathon preparation, long runs don’t necessarily have to be slow all the way. For runners with a solid base, you can add a segment near marathon pace in the latter half so the body adapts to sustained output under fatigue. Tempo runs or cruise intervals follow the same logic: they’re not about briefly sprinting to the limit, but about improving rhythm, fatigue resistance and steady output at a sustainable but not relaxed intensity. But Zone 3 training should be a session that’s been deliberately scheduled, not something that appears randomly during an easy run. Runners need to be clear: why run Zone 3 today, how long will you stay there, will there be sufficient recovery after the session, and will the next day be affected? If these questions have no answers, then the Zone 3 session is meaningless. The real difference between Zone 3 heart rate and the “gray zone” lies in whether the session has a clear purpose, whether the runner actively controls intensity, and whether the Zone 3 portion serves the overall training plan. WhenZone 3 heart rateis scheduled within the plan and serves a clear goal, it’s effective training; when it appears frequently and unconsciously during easy runs, it becomes the so‑called “gray zone.” To avoid unconsciously slipping into the “gray zone,” first clarify the day’s objective before the workout. If it’s an easy run, even if you feel great you don’t need to keep accelerating in the second half. If it’s a tempo, marathon‑pace run or progressive run, entering Zone 3 is part of the plan—the key is steady output, not starting too fast and blowing up later. Second, establish control between pace and perceived effort. On easy runs deliberately hold back the first few kilometers; on long runs keep restraint in the early section and save the effort for the latter half; on tempo runs focus on consistency rather than trying to get faster every kilometer. The essence of the “gray zone” and “junk mileage” is not that Zone 3 heart rate itself is problematic, but that training intensity is mismatched with training purpose.Therefore, what matters is not running faster every time, but returning each session to its proper role so it can deliver its intended value.
Text：Run Wild/Editor：xiaocaiPhoto：The Trail-Running Big Bang/Visuals：Max
