The answer, of course, depends on the individual.
If there is one topic that predominates in any discussion of distance-running training, be it at the elite or the neophyte level, it is the question of how much to run. The intricacies of speed work, debates about what really constitutes a tempo run, and banter about the ideal length and pace of long runs are never far from the core of such conversations, but in my experience, arguments over “ideal” mileage easily reign supreme over all of these put together.
The thing that people need to immediately recognize about this question is that the answer needs to be tailored to individual runners. This is true even of runners of equal demonstrated ability and identical goals. If two similarly experienced runners who have each run 5K in 20:00 using similar, modest training both decide to up their mileage and train for their first marathon with a goal of 3:20:00, this in no way implies that they should undertake identical training loads. It is impossible to tell how much mileage either of them will tolerate and still thrive, or how quickly either should ramp up to whatever training load ultimately proves effective (and this is a fluid ceiling in any case; what is maximally effective this year may prove paltry in subsequent years).
All of the caveats, hedging, and qualifiers in the preceding paragraph underscore the fact that while certain training principles can be deemed universal, at the level of training-plan detail, coaches and athletes must take care not to rigidly emulate someone else’s training. Not should they count on following a published plan to the letter being the most effective strategy for reaching their goals. This becomes increasingly important to keep in mind as the goal race distance increases, because in general, longer races entail heavier training loads and thus greater and greater excursions into the training-volume unknown.
Note that throughout this series, I’ll often refer to “training load” or “training volume” (terms I use interchangeably) rather than mileage. This is because two runners of significantly different abilities training at the same mileage are not really doing the same training at all, for one is effectively putting in a lot more work than the other. A 2:30:00 marathoner running 70 miles a week might average 7:00 per mile in a typical week, meaning that he is putting in a little more than eight hours of running in a seven-day stretch. A 4:00:00 marathoner, on the other hand, is likely to average about a 10:00 pace, so a 70-mile week would require close to twelve hours of running.
Our putative 2:30:00 runner, averaging 7:00 pace, would log 100 miles if beating feet for that amount of time, a state of affairs that may well lead this talented athlete to break down, and almost certainly will in the absence of a gradual build-up. In my experience, this is something that is virtually never taken into account, even when a slower runner has enlisted the guidance of a coach, and has been the downfall of, to give one example, many a hopeful Boston Marathon qualifier.
Individual Training-Load Ceilings: Physiological Vs. Functional
I have divided the factors that determine individual caps on training volume into two categories — physiological and functional — and I’ll discuss them in this order.
I hasten to state that I do not want to mislead anyone into thinking that intentionally seeking his or her personal upper limit and flitting around at that level is the main idea here. At some point, many people will actively seek out their absolute limits — not just in terms of volume, but in terms of intensity, racing frequency, and all sorts of other parameters that can bring runners to a standstill if limits are breached too often or for too long.
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In the end, most folks are more comfortable knowing they tried a given scheme and found they couldn’t handle it than they would be wondering just how much more they could have done. But the idea for most people is to train at something as close to an optimal level as possible without flirting with disaster. Only when people are at least reasonably familiar with what they can likely handle at any given time, and with the main factors determining this limit, can they be confident of approaching this optimum.