This is one of the oldest debates in running, and the honest answer is: both are useful and both are flawed. The right tool depends on the workout.

Pace: The Objective Metric

Pace tells you how fast you’re moving. It’s precise, measurable, and essential for race-specific training. If your goal marathon pace is 9:30 per mile, you need to practice running 9:30 per mile. There’s no substitute.

Use pace for:

  • Interval workouts where hitting a specific speed is the goal
  • Tempo runs at race-specific effort
  • Race pacing strategy
  • Tracking fitness trends over time (same course, similar conditions)

The problem with pace: It doesn’t measure effort. The same 9:00 pace can be a relaxed cruise on a cool morning and a grinding effort in 85°F humidity. Pace doesn’t adjust for hills, wind, fatigue, altitude, sleep quality, or stress. A runner who trains exclusively by pace will almost inevitably run easy days too fast and hard days too easy — collapsing the intensity gap that the 80/20 model depends on.

Heart Rate: The Internal Metric

Heart rate measures the cardiovascular cost of your effort. Unlike pace, it reflects what’s happening inside your body — how hard your cardiovascular system is working to deliver oxygen to your muscles.

Use heart rate for:

  • Easy and long runs, where staying in the aerobic zone matters most
  • Preventing the “too fast on easy days” problem
  • Monitoring recovery (elevated resting HR = accumulated fatigue)
  • Training in variable conditions (heat, altitude)

The problems with heart rate:

Lag. Heart rate takes 15-30 seconds to respond to changes in effort. For intervals lasting 60-90 seconds, your heart rate is always behind — rising after you’ve started and staying elevated after you’ve stopped. This makes HR nearly useless for short, hard efforts.

Daily variability. Your heart rate at the same pace can swing by 5-10 bpm depending on sleep, hydration, caffeine, stress, temperature, and time of day. If you’re rigidly chasing a heart rate number, you’ll slow to a crawl on days when it’s naturally elevated — even though the effort is the same as yesterday.

Cardiac drift. During runs longer than 45-60 minutes, heart rate rises gradually even at a constant pace. Core temperature increases, blood volume shifts to the skin for cooling, and the heart compensates by beating faster. A typical drift is 5-15 bpm over an hour. Your heart rate at mile 8 of a long run doesn’t mean the same thing as the same heart rate at mile 1.

The max HR problem. Heart rate zones are only as accurate as your max heart rate, and most people don’t know theirs. The old “220 minus age” formula has a standard deviation of ±10-12 bpm. Tanaka’s more accurate formula — 208 minus 0.7 times your age — is better, but individual variation is still substantial. If your zones are built on a bad estimate, every workout is calibrated wrong.

The Practical Answer

Easy days: Heart rate zone + talk test. Set a Zone 2 ceiling and stay under it. If you can’t talk in full sentences, slow down regardless of what your heart rate says.

Hard days: Pace when the workout calls for a specific speed. RPE when the workout calls for a specific effort level (“comfortably hard,” “race effort”).

When they conflict: Trust your body. If your watch says Zone 2 but the run feels moderate, slow down — something (sleep, heat, stress) is making today harder than the numbers suggest. If your pace is “too slow” but the effort feels easy, you’re fine.

How Pacewright handles it: The app uses RPE as the primary effort metric because it integrates everything — conditions, fatigue, recovery state — into a single honest signal. Pace and heart rate data enrich the picture when available, but the training load calculation (duration × RPE) works for every runner, in every condition, with or without a watch.