When I was a younger man I used to shake my head in pity when reading the writings of endurance sports experts of a certain age. They tended to repeat the same things over and over, evidently because they had nothing new to say. Because they hadn’t learned anything new about their field of expertise since they were young themselves. Because they hadn’t bothered to try to continue learning. At some point, it seemed, they had simply decided they knew their stuff and stopped seeking out new knowledge.

These aging authorities struck a sad figure in my eyes. As a young man aspiring to expertise in endurance sports, and who therefore payed close attention to new and recent developments in them, I recognized that certain members of the old guard were being left behind, and worst of all, that they failed to recognize their own waning relevance. With the boldness of youth, I vowed never to put myself in such a pathetic position.

Time flies, and now I am a man of a certain age. And, God help me, I feel myself slipping a bit knowledge-wise. Granted, age is not the only factor in my case but also illness. For many years I relied on my own training and racing to stimulate new learning. Long covid has stripped me of my ability to do these things, forcing me to look elsewhere for knowledge. But I can’t blame poor health entirely for my slippage. I can feel my brain slowly transforming from an absorbent sponge into an impenetrable fortress, a normal part of aging. I have less and less patience for technology, for example.

Nevertheless, it remains the case that I don’t want to fall behind, so I pounced when Philip Skiba announced the release of his new book Scientific Training for Endurance Athletes, a review copy of which the author was kind enough to send me. I couldn’t have picked a better way to fill the emerging gaps in my knowledge of endurance training. Skiba is a heavyweight in the field, a technical savant who holds both a medical degree and a doctorate in exercise physiology, has coached a number of elite triathletes, and served as a consultant to Nike’s Breaking2 project.

True to his scientific leanings, Skiba takes a bottom-up approach to explicating how to train for endurance racing, going from physiology to intensities to workout types to periodization. Personally, I prefer a top-down approach that starts with real-world best practices, as I believe that context is everything and that, for this reason, it’s impossible to deduce best training practices from physiology. That being said, Skiba’s approach serves mainly as a pedagogical device, and it does an effective job of making sense of endurance fitness and training objectives. Indeed, Skiba has a special gift for making science understandable to the layperson, of which I am one. My favorite passage in the entire book is his house metaphor of endurance fitness, which goes like this:

The foundation is your basic strength and resilience. The floor is your endurance capacity, and the ceiling is the critical power/speed. The roofline is your VO2max. The top of the roof is your peak power output. Let’s imagine that your current marathon speed (usually very close to lactate threshold) is equal to your height. You walk into your house, and mark your height on the wall. With time, as you train, you grow taller. In the beginning, the whole house grows with you. However, what you will find is that with time you will begin to bump your head against the ceiling. You need to do some specific renovations on the house to raise the ceiling so that you can continue to grow. However, what you will quickly find is that you are squeezing the ceiling too close to the attic above. Eventually, you need to raise the attic as well.

Skiba's House Metaphor
Skiba’s House Metaphor

Overall, I found Skiba’s book reassuring. While I learned a lot from reading it, including how to calculate optimal interval numbers for individual athletes based on their current fitness, for the most part it confirmed what I already knew and left me feeling I haven’t yet fallen as far behind as I had begun to fear. The book also heightened my sense that, increasingly, the folks who really know what they’re talking about with respect to endurance training are speaking the same language. With his focus on the power-duration curve, which represents fitness in terms of how long an individual athlete can sustain a given power output or velocity across the spectrum of effort levels, Skiba approaches the problem of developing race-specific fitness through the same lens as the likes of Stryd, Alan Couzens, and yours truly.

Scientific Training for Endurance Athletes isn’t for everyone, but it has become the very first book I recommend to performance-minded athletes who want a thorough and up-to-date understanding of how endurance training works and how to make it work best. Already I’ve purchased a copy for Coaches of Color Initiative apprentice Jessica Schnier, added it to the Resources section of the forthcoming 80/20 Endurance coaching textbook, and convinced a couple of the athletes I coach to order it. And you can soon expect Dr. Skiba to be a guest on the 80/20 Endurance podcast, where we’ll dive much deeper into his impressive work (and I don’t just mean impressive for an old guy!).



Trail running is becoming more and more popular—statistics say so. But I don’t need statistics to know that increasing numbers of runners are taking to the trails. I can tell by the emails I receive from advice-seeking athletes, a rising percentage of which are sent by trail runners.

The question that is most frequently asked by this cohort is a version of the following: “I do most of my training in the mountains and I find it difficult to keep my heart rate in Zone 2, especially on steep climbs. How do I obey the 80/20 Rule as a trail runner, or does it not apply to me?”

In case you are unaware, the 80/20 Rule is the idea that endurance athletes in all disciplines and of all ability levels gain the greatest amount of fitness when they do approximately 80 percent of their training at low intensity. On my 80/20 intensity scale, the top of Zone 2 corresponds to the upper limit of low intensity, so in practical terms, applying the 80/20 Rule means keeping your heart rate and/or pace and/or power below the top end of Zone 2 four-fifths of the time when running.

Due to the effect of gravity, runners must slow down to maintain the same physiological intensity when running uphill. Athletes with low to moderate levels of fitness may even have to dial all the way back to a walk to stay in Zone 2 on steeper climbs. Before I move on to talk about what these individuals should do to avoid falling into the all-too-common “moderate-intensity rut” as trail runners, let me first point out that runners at higher levels of fitness need not make any special modifications to their training as trail runners to stay in line with the 80/20 Rule.

I’ll use myself as an example. At my present level of fitness, my Zone 2 tops out at about 6:54 per mile. According to a certain online calculator, the effort level that is associated with running 6:54 per mile on level ground is equivalent to the effort level associated with running 9:32 per mile on a steep hill with a 10 percent gradient. So all I have to do to avoid creeping into moderate intensity in a hilly run that is intended to be done entirely at low intensity is keep my pace slower than 9:32 per mile on 10 percent inclines and make similar adjustments on hills with other degrees of slope. It’s just a matter of being aware and disciplined.

Now, I grant that most runners cannot ascend a 10 percent hill in Zone 2 without shifting to walking. So, then, what should you do if you’re in this group? My first suggestion is that you use a run power meter such as Stryd to monitor and control the intensity of your runs. This tool will give you a more reliable picture of how you are distributing the intensity of your training than will either pace or heart rate. Unlike your pace at the top end of Zone 2, your power at the top end of Zone 2 doesn’t change with topography. If your Zone 2 power tops out at, say, 220 watts, it does so regardless of whether you’re running uphill, downhill, or on level ground.

It’s true that your Zone 2 heart rate range also does not change with topography, but the trouble with heart rate is that it lags behind changes in intensity, so when you’re running on highly varied terrain your heart rate monitor is continually giving you yesterday’s news, so to speak. It can work, but not as well as a power meter.

My other bit of advice is that you match your workouts with your training venues so that you avoid spending more than 80 percent of your training time above Zone 2. One way to do this is to avoid challenging trail routes when doing runs that are intended to be done entirely at low intensity. A second, and complementary, way to achieve the same objective is to budget “unavoidable” time above Zone 2 into your weekly allowance of moderate- and high-intensity running. For example, suppose you like to run up a mountain and back down once a week and you’re above Zone 2 during the ascending portion of the run no matter how slow your pace is. Let’s supposed further that it takes you about one hour to get to the top and 35 minutes to come back down. There’s no reason you can’t include this workout in your weekly training schedule provided that the 60 minutes you spend above Zone, combined with any other moderate- to high-intensity running you do during the week, does not represent more than 20 percent of your total training time for the week.

There, I’ve taken away any and all excuses you might have had for falling into the moderate-intensity rut as a runner who trains primarily on trails.

Dear David W,

I’m going to buy your plan for the level 3 half marathon plan. I have a race (in Madrid, Spain) on April 8 which by my calculations will have me starting Dec 31/Jan 1… I had a bike wreck in a triathlon a few months ago that resulted in broken bones, concussion, etc. So I’m just now starting to get back into where I can work out 4-5 times a week. So I’m on pace to be at least to the proper starting place for your program.

I am a gadget guy. I supported Stryd on Kickstarter and upgraded to the footpad at the first opportunities. I record my power on my runs, but it is just another interesting bit of data that I review at the end of some of my runs. So I’ve never really done much with it. That is why I’m so intrigued by this [the 80/20 power-based run plans].

So that leads me to my questions: When should I do the power test? I would assume fairly close to the start of the program so that my numbers are pretty accurate for that time. As my training increases and my thresholds change, will I be doing more tests during the training plan?


Dear DH,

I wish you the best of luck on your recovery.

You are exactly right, particularly where you are coming back from low fitness, your power thresholds will constantly change (improve). The run plans have RT (run tempo) workouts scheduled every recovery week, which is every 3rd or 4th week. Those RT workouts are designed to confirm or re-establish your zones. Therefore, you’ll be testing regularly anyway with an 80/20 plan.

It would be good to do a test before your start your regular training. We have a suite of tests in our Intensity Guidelines for Running document on our 80/20 Resources page. Some of the tests are brutal (30-minute time trial) and some are easier, if slightly less accurate. I would do the easy test at first. Make sure to see the section on RT workouts at the bottom of that guide.

Also consider that HR zones will change very little. Your output for a given HR will, but early on, consider using HR as a secondary (or primary) measure of intensity until your power zones level off.

Finally, we offer a Level Guarantee. If you buy the Level 1 plan and find that 8 weeks later it is too easy, come back to me and I’ll get you the L2 plan for free. This applies to leveling up or down on all of our plans.


Matt and I periodically publish anonymously your inquiries to us, particularly when the answer may benefit the community. Have a question about 80/20 training or training in general? Feel free to e-mail me. David W.

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