There are many runners who are like golf carts fitted with the engine of a Ferrari Testarossa. They have the genetic enzymatic potential to develop the oxygen-carrying capacity and end-organ (i.e., muscle) oxygen-processing capabilities to become much-better-than-average, even elite distance runners. But alas, while the engine is nonpareil, the chassis is lacking, and sometimes there isn’t a lot to do about this.

People in good aerobic shape look more or less the same from a distance or even up close, but every runner is put together a little differently. Between leg-length discrepancies, unfavorable lower-leg-meets-knee (or foot) configurations, and too many other human variants to name, it seems likely that for the majority of runners, running volume is ultimately limited by the sheer effects of biomechanical stress on their bodies.

One thing to note is that what may feel like a permanent blockade to running more may really be a temporary ceiling resulting not from true structural limitations, but from muscle breakdown and attendant soreness. The amount of running that had me feeling as if I’d been put through a farm combine at age 20 seemed like a modest total a decade hence. But muscle soreness and fatigue can be readily distinguished from frank injury, a clear propensity toward certain overuse injuries, and chronic problems that clearly spell a need for volume moderation.

Understanding Body Types

Obviously, this distinction takes time to clarify; someone who suffers a tibial stress fracture while running 40 miles a week on asphalt at age 25 may or may not be able to eventually maintain or increase this workload by switching surfaces some or all of the time (something that is beneficial regardless of tendency toward injury); the stress fracture may turn out to be “one of those things” that never recurs or it may herald the onset of a pattern. There’s just no way to tell in advance how well your body is likely to bear the burden of increased demands once a runner has reached a comfortable plateau and is prepared to move forward.

Body type is typically regarded as a reliable indicator of how a runner is likely to fare at higher workloads. As a rule of thumb, a 220-pound former linebacker is probably not going to be able to run as much as a diminutive, natural ectomorph who checks in at 130. This rule, however, has plenty of exceptions, for the way a runner lands and distributes his impact stresses is at least as important as how big he is. A light, quick runner with an exaggerated forefoot push-off may experience chronic calf problems at higher workloads (or when doing lots of speedwork), whereas our 220-pounder may shuffle more than stomp and therefore be able to put in a surprising amount of volume.

I have seen runners with birdlike frames who are perennially prone stress fractures as well ponderous-looking specimens who never seem to suffer the slightest twinge. Footwear and choice of running surface can go a long way toward influencing these outcomes, but the most important factor seems to be how a runner picks them up and puts them down.

Runners who are limited to workloads they find unsatisfactory by their stride mechanics have other options for furthering aerobic development, and articles about cross-training can be found all over the Internet.

Measuring Recoverability

Training itself actually results in a breaking down of bodily tissues; it’s the rebuilding phase, when muscle cells are reassembled and certain enzymes increase in concentration, in which fitness gains actually occur. It follows that no amount of training by the most durable runner will be effective training unless sufficient adaptations occur between training bouts.

Recoverability, as with the connective-tissue adaptations that occur throughout years of ever-increasing training volume, is impossible to measure directly. But there are clues pertaining to both its enhancement and its ultimate limit within any one body.

Runners may recall their earliest days in the sport, when a run that produces no detectable damage in the present left them sore for two or three days and thus hampered further training. And those who have been fit but then experienced extended layoffs know all too well that one of the biggest issues in returning to the fray is dealing with feeling beat up all the time in the beginning of the “comeback” phase. Part of getting fitter — be it going from zero to some modest level or from reasonably fit to the best shape of one’s life — is bouncing back more quickly between sessions.

But as with all physiological traits, everyone’s maximum potential recoverability is different, with the implication that some people simply can’t train as hard (or as much) as others despite being equal in every other measurable respect. This principle is underscored by the fact that distance runners have been known to take certain types of anabolic steroids (nandrolone perhaps being the most common in the past couple of decades) which do not promote bulk, but speed recovery time by promoting rapid muscle repair. These banned substances don’t make anyone faster per se; instead, they allow for harder and more effective training.

There are ways to ensure that your recoverability is close to its potential maximum, however. For one thing, running on forgiving surfaces such as grass and dirt (or even treadmills) entails less pounding and shortens recovery time. Being careful to keep glycogen stores above ground level by re-fueling as soon as possible after every run is also key, as is—at least according to some research—a high-protein diet. Finally, adequate sleep is necessary if you want to take advantage of the running you’re doing rather than simply log miles.

Cortisol, an hormone produced in the adrenal glands in response to stress that has a variety of “anti-recovery” effects, is found in higher levels in sleep-deprived people; this is only one of the reasons why you’re not going to recover as well if you’re not getting enough sleep. Simply put, you can log as many miles as you like, but if you’re only getting four or five hours of sleep a night, you might as well be tossing those miles down a well.

Yes, you’ll be punching the clock, putting in miles and burning calories, but you won’t be getting any more race-ready if you aren’t recovering.