This question gets at the heart of what makes running training so delightfully complicated. We all know that most of your training (likely around 80 percent, with variance based on background) should be easy. But, sometimes, easy running is close to impossible based on hills, heat or training partners. When the math equation breaks down, training theory is pushed to its limits. And all the fun happens at the limits.
First, a step back. What is easy running? Loosely, “easy” can be approximated as any effort below aerobic threshold, the range when the body switches primarily from using fat for fuel to primarily using carbohydrates. Exceeding aerobic threshold involves a slight pick-up in breath rate, maybe some minor resistance in the legs. You can even approximate it with a heart rate monitor and a field test. I prefer the “Friel Test”—a 30-minute hard effort (after a warm-up), taking the average of the last 20 minutes to approximate your lactate threshold heart rate, and multiplying that by 0.84-0.89 (lower for high-mileage or advanced athletes, higher for low volume or beginner athletes) to get a feel for a loose cap on your easy runs.
That field test is a more accurate way of getting at aerobic threshold than the more commonly used “MAF” approximation. MAF applies a simple formula—180 minus age—and uses that as a hard cap for easy days. While it’s often close to the physiological reality, heart rate depends heavily on genetics and training background, so the equation can be like deploying a sledgehammer when a scalpel would be best. Strict MAF approaches can involve running under that cap exclusively. The theory is that the body gets more efficient with lipid metabolism while also improving aerobic endurance via lower stress, increased capillaries around working muscles and sustainable musculoskeletal strengthening from consistency.
But there’s a problem. One of the most common questions I receive is full of exasperation. “I have been running easy all the time, and everyone says my easy pace should get faster, but I keep getting slower. What’s wrong with me?” That lament is strikingly prevalent, and I think it gets back to a misconception about easy running more generally. Improving the aerobic system is just one part of translating oxygen into speed.
The aerobic system is applied through the interaction of the musculoskeletal and biomechanical systems, and those systems need to be stressed too. Otherwise, an athlete might be left with a boss heart and badass lungs, but legs that translate aerobic efficiency into inefficient output. I would argue that most of the MAF success stories require massive volume and natural talent (or both). You can’t go fast by going exclusively slow unless genetics or background make the aerobic system the primary limiter—there needs to be some natural predilection toward speed, or some speed reinforcement along the way.
In other words, we are not just lungs. We are not just lungs with legs. We are lungs with legs and a nervous system tying them together, applied via complex neuromuscular and biomechanical pathways. And that usually requires easy running to leave space for higher output days that exceed strict guidelines, especially when the strict guidelines require an athlete to walk, which won’t have too much musculoskeletal crossover with running past initial adaptations.
That’s why I encourage athletes to practice running more hills over time, even on easy days. The impact forces are reduced on uphills, so injury risk is lower. The biomechanical strain of uphill running likely improves running economy on all terrain. The musculoskeletal system can adapt to that stress to make it possible to do more hills at a true “easy” effort later.
And the most important takeaway—there is no magic barrier when an easy run becomes unproductive. Increasing the difficulty of some easy runs to withstand uphills or heat without stopping or walking is only going to enhance the aerobic system as long as an athlete isn’t at their stress limits to begin with. To add some training terminology to it: it’s OK to progress easy run effort to steady (the grey area between aerobic threshold and lactate threshold) in challenging conditions and when you feel good, as long as you’re not causing muscle breakdown or excess production of the stress hormone cortisol. You’ll know to back off if you’re not recovering.
Of course, some hills will still be inadvisable or impossible to run, particularly when tired. But with a balanced training approach focused on a relaxed framework for “easy” running, with time, you’ll be able to run more of the hills. And that’s one way for a trail runner to level up and let their true talent shine.