The important thing to keep in mind about training volume is that the level a given runner can handle is a fluid quantity.
That is, a runner who remains healthy most of the time can very reasonably expect to be able to effectively train more and more as the years pass. (My use of “effectively” here implies that handling the training means not merely surviving it, but becoming stronger over time and thus benefiting from the workload.) I’ve seen this happen even during periods of performance stagnation; in fact, an increase in workload is commonly accompanied by lackluster racing, sometimes with steps backward, after which adaptation is achieved and a performance breakthrough occurs.
Of course, runners have to be realistic about whether a jump in volume that results in excessive fatigue and poor competitive efforts is truly something bound to result in new reservoirs of strength or whether it is simply a matter of chronic overshoot that can only end badly. This is why it is always useful to have a second set of trusted eyes on your training, particularly when making significant changes that entail greater demands.
I cannot recall a single instance in which, all else being equal, a runner who starts modestly and diligently and wisely indulges in training increases over a period of years fails to become a faster competitor. Note the caveats: Someone who’s running 80 miles a week as a precocious high schooler may never thrive on much more than that, and someone who discovers running at age 30 and ramps up from 25 miles a week to 75 within a year may quickly find that his or her race results are wanting, assuming he or she is still upright.
But it seems to be a fact that if a runner slowly increases volume — and alas, I can’t pin a number on this, and neither can anyone else — over a period of years, remains healthy, and is careful to take some honest down time after periods of especially intense training and racing, he or she will invariably become faster. This in no way implies that improvement is unbroken by setbacks; as with most things in life, a graph of achievement level vs. time much more closely resembles a jagged “sawtooth” trending ever upward than a straight, unbroken line.
To take this from the abstract from the concrete, I’ll offer a real-life example. A 14-year-old with a modest background in sports takes up cross-country in the ninth grade. In his first season, he averages perhaps 25 miles per week and takes his 5K time from 21:00 to 19:30. Over the winter and spring his training is similar. Over the next three years, his summer training for cross-country increases steadily from about 35 miles a week before his sophomore year to about 50 to 55 before his senior year. He leaves high school with a fastest 5K of just under 16:00.
In college, he reaches a one-week high of 80 miles in his freshman year and averages close to 70 for extended periods of time. He improves at first, but is set back by a bout with iron deficiency, and by the time he is a junior, various distractions have rendered running competitively a chore and he elects midway through his junior year to not compete for the varsity team anymore.
After a couple years of casual running he finds himself with renewed motivation. Preparing for his first marathon, he increases his mileage over a period of almost a year from 60 miles a week to close to 100. He does at least half of his training on grass. He performs respectably in the marathon with a 2:39 debut, and after upping his mileage to 120 a week over the next few months he promptly incurs a metatarsal stress fracture.
Once this heals, he embarks on a quest to see how close to 2:20 he can get. He runs 2:33, 2:30, and then 2:26 over a four-year span. At 31, having endured as many setbacks as he has successes, he finds himself capable of consistent 110-mile weeks and runs 2:24. The following year, he averages over 100 miles a week with no injuries, but does not race especially well. His goal of running under 2:22 the next year, when his mileage is similarly high and his overall racing is satisfactory, goes unmet, but the year after that he, at 34, runs a slew of personal bests at other distances.
Few runners will ever reach these mileage totals, but the idea here is to illustrate how subtle year-to-year increases in volume can ultimately translate into the ability to reap benefits from training loads that would have crushed the same runner in his or her younger days. The resilience of connective tissue and the musculoskeletal system as a whole can’t be quantified, but it unquestionably becomes greater over time with consistent training.
Every runner has a different limit in terms of this (and every) sort of adaptation, but it is real. In theory, a young man of 20 who has been running quite a bit for five years ought not to be as sore from back-to-back 14-milers as he would be 15 years later. Yet speaking from my own experience, even as I approached my 40th birthday, I could easily handle training loads I could not have dreamed of accommodating two decades ago even though my speed had slipped.
The main lesson? Whatever volume you can do today may be a poor indicator of what you might be able to do in one, three, or 10 years. This is largely age-dependent, of course; if you take up running in your forties, it’s unrealistic to expect that you will progress up the volume scale as prodigiously or for as long as someone who takes up running in his teens.
But the general principle holds: Carefully and gradually boosting your training volume — and at the end of this series I’ll offer general strategies on how to best accomplish this — so as to bring it in line with your effective physiological limits virtually always yields both an “unusually” late lifetime peak and the ability to retain peak racing fitness for a greater-than-average number of years.