If I were to ask a randomly-chosen runner what they were planning to do for training tomorrow, the answer would probably be something like “6 miles easy” or “400-meter repeats” and not “50 minutes easy” or “80-second intervals.” Most runners, in other words, measure their runs by distance. This practice is so familiar that the alternative practice of time-based training (“I ran 5 hours and 28 minutes last week”) might seem so weird to you that you’d have to translate the hours and minutes into your native tongue of miles and meters to be able to put it in context. Yet in running’s close cousin, cycling, time-based training is the norm.
Don’t worry — I’m not about to tell you to stop tracking your mileage. I am, however, going to make the argument that time should be your primary consideration in planning your training. Why? Because your body operates on time; distance means nothing to your organs, cells, and genes. What I mean by this is that running is no different from other stimuli, such as sleep and sun exposure, that affect the body. The strength of its effect is determined not by how far you go but by how long you do it.
I know what you’re thinking: “Aren’t time and distance two sides of the same coin?” After all, the farther you run, the more time it takes, and the more time you spend running, the farther you go. This is true to a point, but only to a point, and to the extent that it’s not true, the difference between time and distance is crucial. Ten minutes of running might equate to 1 mile in one instance and to 2 miles in another. The goal of training is to subject the runner to the right amounts of the right stimuli, and as I will explain in this article, it is time rather than distance that determines the right amount.
The Intensity Question
Whether you’re a newbie runner or an elite runner or anything in between, your training needs to regularly target specific physiological intensities to produce optimal results. What’s interesting about these intensities is that neither talent nor training has much effect on how long an individual athlete can sustain them. For example, a 4-minute miler and a 7-minute miler are both able to sustain an all-out sprint for 6 or 7 seconds before they begin to lose steam.
Another example — one that is more germane to endurance training — is VO2max intensity. As you probably know, VO2max is the exercise intensity at or above which your body consumes oxygen at the highest rate it is capable of. In running, VO2max velocity (vVO2max) is the slowest running speed at which your body will encounter this limit. And the maximum duration an individual runner can sustain VO2max velocity is called the tlim (time limit). You might expect that fitter runners would be able to sustain their individual vVO2max longer than runners who are less fit, and that training would increase the sustainability of this speed in all runners — but research has shown that this is not the case. Whether fast or slow, fit or unfit, most runners have a vVO2max time limit of about 6 minutes.
The practical implication of this fact is that if you want to give two runners of different ability a VO2max-targeting workout that is equally challenging for both, that workout needs to be time-based.
Consider two runners, one of whom has a vVO2max of 5:00 per mile and the other of whom has a vVO2max of 7:00 per mile. If both runners are given a distance-based workout consisting of 6 x 800 meters @ vVO2max with jog recoveries, the faster runner will spend a total of 15:00 at an intensity they could sustain for roughly 6 minutes straight, while the slower runner will spend a total of 21 minutes at the same intensity. Clearly, this workout, which on the surface appears to be the same for both runners, is actually far more challenging for the slower runner.
I get it: You’re accustomed to measuring runs by distance, all of your running friends do it, and quite frankly, you like it. That’s okay. If you don’t wish to abandon distance-based training entirely, just plan by duration and convert to time. Using the example above, two and a half minutes is a sensible duration for vVO2max intervals. To perform a distance-based workout at this intensity, just calculate how far you will run in 2.5 minutes at your vVO2max and make the intervals that length. Suppose your vVO2max is 6:30/mile. At this pace you will cover 611 meters in 2.5 minutes, so in this case 600 meters would be a good distance for your vVO2max intervals. A runner with a vVO2max of 9:00/mile would go about 450m (1609 meters / 9 minutes = 179 meters/minutes x 2.5 minutes = 447 meters) in the same workout.
Race Pace and Distance
The exceptions to this principle are runs intended to give you practice in running at your target race pace for a particular event. Slower runners race at a lower relative intensity than faster runners do, and for this reason workouts intended to give runners an opportunity to practice running at race pace can and should be distance-based. For example, a good workout design for 10K pace practice is 10 x 1K at 10K pace with 1:00 rests. This workout is equally challenging for the 30-minute 10K runner and the 60-minute 10K runner in precisely the same way that actually racing a 10K is equally challenging for these two runners.
What about longer races? A marathon is 26.2 miles for runners of all abilities. If a 2:45 marathoner and a 4:15 marathoner both cap their long runs at 3 hours, the slower runner will be significantly less prepared to go the distance on race day. There are some coaches who say, in effect, so be it. But I have found that the confidence that back-of-the-pack runners gain from covering at least 18 miles at least once in training is worth the risks that come with multiple hours of pavement pounding. I choose therefore to prescribe distance-based long runs in marathon training.
Trail ultramarathons are another matter, though, for two reasons. The first is that nearly all ultras take place on trails, hence nearly all long runs in ultramarathon training should also take place on trails. But trails vary wildly in their degree of difficulty. The same runner may cover 14 miles in 2 hours on rolling fire roads and only 10 miles in the same time on technical mountain singletrack. Running the same distance on the two different types of terrain would result in nonmatching challenges.
The second reason it’s advisable to do time-based long runs in ultramarathon training is that it takes most runners more than 5 hours to complete most ultras, but no human is capable of adapting beneficially to training runs lasting longer than about 5 hours. Anything longer won’t make you any fitter; it will only break you down. Thus, faster and slower runners alike will be best prepared for their next ultra if their long training run progression tops out at or near 5 hours. Obviously, faster runners will cover more distance in that time, just as they will in the race itself.
The Volume Question
The typical elite runner runs between 100 and 120 miles per week. It takes between 11 and 14 hours for such a runner to cover this distance. If you tried to run 100 or miles per week, you would probably get injured. But could you run 11 or more hours per week? Most recreational runners could, provided they built up to it properly.
What is true of running intensity is also true of running volume. The amount of fitness you gain through weekly running volume is determined by how much time you spend running each week, not by how much distance you cover. If your average pace per mile in easy runs, tempo runs, and interval runs combined is 8:45/mile, then you will match the volume of an elite runner averaging around 6:00/mile by running about 75 miles per week.
That’s still a lot of running, and my point is not that every runner should run this much. My point, rather, is that if you want to model your training after the pros, you need to measure training volume in time instead of distance to put in a similar training load.
A secondary benefit of doing so is that it will help you avoid the trap of running too much in order to achieve meaningless round-number thresholds such as 50 miles in a week. Many of Kenya’s elite runners don’t bother tracking weekly volume, not by distance and not by time either, staying focused instead on doing what’s needed day by day. That’s worth noting, because I hear some Kenyan runners are pretty good.