The pace on your watch can look perfectly ordinary while the run itself is costing you more than usual. That gap matters: follow the pace blindly and an easy day can become too hard, or a workout can miss its intended effort. Repeat that mismatch, and you can undermine the balance between training and recovery that helps build race fitness.
Say you head out for an easy run at your usual pace. A mile or two in, your breathing is labored and your heart rate is higher than expected. Pace alone says nothing has changed. Your body is telling a different story. Heat, hills, fatigue, and stress can all make a familiar speed feel harder. The tricky part is knowing which signals to trust and what to do with them.
Ultramarathon runner and endurance coach Nick Farr, founder and head coach of P3 Fitness, sorts those signals into three groups: performance output, biological load, and subjective feedback. His point is simple, and more significant than it may first appear: a useful read on a run comes from putting the numbers, your body’s response, and your own sense of effort together.
Here’s how that framework can help you judge whether a run is serving its purpose, adjust when conditions or fatigue change the equation, and build fitness with more consistency.
Why Your Run Data Needs Context
“No single metric tells you the whole story [of a run],” Farr says. Pace tells you how fast you went; it can’t tell you how demanding that speed felt or how your body handled it. If you're working toward a training goal, that distinction is practical: the same output can come at a different internal cost from one day to the next.
Farr’s three categories each answer a different question:
- Performance output: What work did you do?
- Biological load: How did your body respond?
- Subjective feedback: How did the effort feel?
Read together, those answers help you judge whether a session matched its purpose and whether conditions, fatigue, or another factor calls for a change.
Performance Output: What the Run Produced
Performance output is the work you can measure: pace, speed, distance, time, elevation, cadence, stride length, and power, Farr says. These figures show how far and fast you ran, how long you were out, and how much climbing you did. Compared across sessions, they can reveal changes in performance.
But a pace number doesn’t explain the conditions behind it. You might hold your usual speed on an easy run while heat, hills, or fatigue turn that effort into something much harder. Without the other signals, it’s easy to mistake matching pace for matching effort.
Biological Load: How Your Body Responded
Farr also calls biological load internal or physiological load. It includes measures such as heart rate, breathing rate, oxygen consumption, and core temperature. They indicate how your body is responding to a given pace, environment, or training block.
Imagine running in 90-degree July heat. You may manage your normal pace, but your heart and breathing rates can rise as your body works harder to sustain it. Those readings add context that pace can’t supply: the output is familiar, while the cost has increased.
You don’t need to monitor every possible marker, Farr says. Start with one you can readily track, such as heart rate, and notice how it relates to the effort you intend to hold. Comparing similar runs over time can show when a familiar pace is asking more—or less—of you.
Subjective Feedback: Your Read on the Effort
Subjective feedback is what you feel. You can rate the effort on a 1-to-10 scale using rate of perceived exertion (RPE), check whether you can hold a conversation on an easy run, or notice if your legs feel heavy near the end of a speed session. The measure can be as straightforward as your own judgment of how the run is going.
That personal read can catch a change that your watch won’t. A pace that usually feels comfortable may feel taxing when you’re carrying fatigue or stress. Take that change seriously: it’s a reason to check the other signals and consider adjusting.
Farr has newer athletes run by perceived effort alone at first, so they can learn what different intensities feel like. After a few weeks, he introduces measures from the other categories, including pace and heart rate. Those readings then help athletes connect their felt effort to the demands of each workout.
The Value Comes From Combining the Three
Each category offers useful information, but Farr recommends tracking at least one measure from each during a run or workout. His starting rule: when two of the three categories point the same way, use that agreement to guide your decision in the moment.
Return to the easy-run example. Your pace is on target, but your heart rate is elevated and the effort feels unusually tough. Biological load and subjective feedback agree that the run is costing more than expected. Slowing down may better serve the goal of an easy run: accumulating mileage while staying comfortable, supporting recovery, and building your aerobic system.
Signals can also point the other way. On a cool day, your planned pace might feel easier than expected while your heart rate sits below its usual level. Depending on what the workout is meant to accomplish, Farr says, that combination could support a modest increase in effort.
The workout’s purpose is the reference point. A slower pace doesn’t automatically mean a session has lost value if your biological markers match the intended effort. “On any given day, the pace may look slightly different, and that’s totally fine,” Farr says. “That’s just a shift of the output, not a shift of the value of the workout.”
How to Apply the System
There’s no need to spend a run staring at your watch or parsing every sensation. Farr’s approach starts with knowing the session’s purpose, checking how the signals fit together, and using what you learn to inform later training.
- Before the run, define its purpose. Know what the session should accomplish and what effort it calls for. For an easy run, decide what “easy” should feel like and which signals can help you keep it controlled.
- During the run, compare the signals. Check your output, your body’s response, and how the effort feels. If they don’t line up, consider whether conditions or fatigue explain the gap, and whether changing your effort would better match the session’s aim.
Farr keeps pace and time together on one watch screen and heart rate on another. That lets him check the numbers when he chooses while still noticing how the effort feels.
- Afterward, write down what happened. Review the output and biological data, then note how the run felt and what conditions you faced. The record gives future sessions a useful comparison. If you repeat a speed workout from a cool spring day in summer heat, for instance, your log can help you recognize that a slower pace may be needed to match the intended effort.
- Look for patterns over time. Farr suggests revisiting a familiar route at a consistent pace from time to time and comparing heart rate and perceived effort. If you usually choose a new route, try returning to the same one monthly. Similar conditions make the comparison more useful. If the pace begins to feel easier or comes with a lower heart rate, that may be a sign that your fitness is improving.
