By Dr. Brian Atkinson

The single most common mistake new marathoners make is running their easy runs too fast. A marathon is roughly 98% powered by your aerobic system — the "oxygen-burning" engine in your muscles. When you run at an easy, conversational effort, your heart and lungs deliver oxygen to the mitochondria inside your muscle cells, which burn a mix of fat and carbohydrate to produce steady, long-lasting energy. Running easy, over and over, is the signal that tells your body to build more mitochondria, grow more tiny blood vessels to feed them, and strengthen your heart. This is your "aerobic base," and it is what carries you through mile 20 and beyond. Even many elite marathoners do about 80-90% of their running at an easy effort.

When you push the pace too hard — especially on long runs — your body increasingly leans on a faster but much less efficient backup system: anaerobic glycolysis. This system burns through your limited stored carbohydrate (glycogen) quickly and produces byproducts linked to muscle fatigue. It has its place in short, deliberate speed workouts, but on easy and long runs it is not what we want. Running too hard turns every run into a moderately hard effort, which means more fatigue, slower recovery, higher injury risk, and less of the aerobic development you are actually training for. Slowing down on easy days lets you show up fresh and run more consistently, and consistency is what builds a marathoner.

If running slowly enough feels impossible at first, walking breaks are completely fine! A run/walk approach (for example, run 4 minutes, walk 1 minute) keeps your effort in the right range while you build fitness, and research shows run/walk marathoners can finish in similar times to those who run the whole way, with less muscle pain and fatigue afterward¹. As your aerobic system improves, you will notice you can run faster at the same easy effort. This is one of the most satisfying parts of aerobic training and you will notice improvement quickly!

How do you know you are running "easy"? Here are a few simple ways to estimate it:

• The talk test: You should be able to speak in full, comfortable sentences. If you can only get out a few words between breaths, slow down (or walk).
• The Maffetone (MAF) method: Subtract your age from 180 for an approximate upper heart rate limit on easy runs. Subtract 5 more if you are new to exercise, coming back from injury or illness, or take regular medication; add 5 if you have trained consistently for 2+ years without problems. Example: a 40-year-old beginner would aim to stay below about 135 beats per minute.
• Percent of max heart rate: Easy running generally falls around 65–75% of your maximum heart rate. A rough estimate of max heart rate is 220 minus your age, though this can be off by 10 or more beats for any individual.
• Rate Perceive Exertion (RPE): On a 1–10 effort scale, easy runs should feel in the 2–4 range — relaxed enough that you finish feeling you could have kept going.

Heart rate formulas are estimates, not rules. If your watch and the talk test disagree, trust how you feel and your breathing. And if you take medications that affect heart rate (such as beta-blockers), have a heart condition, or notice chest pain, unusual shortness of breath, dizziness, or fainting while running, check with your doctor before following any heart rate target.

¹Hottenrott K, Ludyga S, Schulze S, Gronwald T, Jäger FS. Does a run/walk strategy decrease cardiac stress during a marathon in non-elite runners? J Sci Med Sport. 2016. PMID: 25467199.

About the Author:Dr. Brian Atkinson is a primary care and sports medicine physician at MAHEC Sports Medicine. He brings specialized expertise in family medicine, lifestyle medicine, and sports medicine to his practice. With a focus on musculoskeletal injuries and preventive care for active individuals, he works closely with his patients to develop personalized treatment and recovery plans. Dr. Atkinson is particularly passionate about helping patients of all ages maintain wellness, mobility, and function throughout their lives. He is also team physician for UNC Asheville Athletics, Warren Wilson College Athletics, and medical director for multiple road and trail running races. He takes pride in educating his patients on the importance of physical activity, nutrition, and overall well-being as part of a comprehensive approach to health. Brian is also a long-time competitive runner, including running collegiately at Duke University and qualifying for the 2016 U.S. Olympic Marathon Trials.