At SQUADRUN we usually prescribe intensity using pace, minutes per kilometre. Most sessions in your week will come with a target pace range, usually long slow distance, sometimes recovery, marathon pace, threshold, and the other usual suspects. The reason is boring but practical, most runners own a GPS watch, pace is easy to see, easy to follow, and easy to repeat.
Pace is also easy for us to set accurately because we can reverse engineer it from benchmarks you already understand, your current 5k, 10k, half or marathon best. Those times let us estimate your current fitness, then translate that into training speeds that match the intent of each session. On flat, predictable ground, pace is the cleanest steering wheel you can hold. Road running is basically a treadmill with scenery, pace behaves nicely there.
The moment you leave the flat stuff, pace starts lying to you. Hills and trail turn “5:15 per k” into a joke, because the cost of movement changes. One kilometre up a climb is not the same as one kilometre down a rocky goat track. On technical terrain, you can be working hard while moving slowly, or moving quickly downhill while your legs are getting punched in the quads. Pace is a good servant on the road, and a complete menace in the bush.
So you might think, sweet, I’ll use heart rate instead. Heart rate is useful, but it’s also a bit of a drama queen. Heat, dehydration, poor sleep, caffeine, stress, accumulated fatigue, all of it can push your heart rate around even if the effort is the same. Then you’ve got cardiac drift, where heart rate creeps up over time at a steady output, plus the messy reality that tired legs can’t produce the same force. When the muscles are cooked, you physically cannot apply the same load, so the oxygen demand can drop even though it feels grim. Heart rate is a decent compass, but it sometimes points at whatever it feels like today.
Running power can help, but it has its own quirks. Uphills tend to inflate it, downhills can make it look laughably low, and it often struggles to account for terrain complexity. Picking your way through roots, rocks, mud, sand, tight corners, that can be high effort with low mechanical output. Power might tell you you’re cruising while you’re actually doing trail parkour and quietly panicking.
That’s why there’s no single guiding star. In the real world, especially off road, the best anchor is RPE, your rating of perceived exertion, basically a 1 to 10 scale of how hard the work feels. It’s the only metric that automatically includes everything, gradient, footing, fatigue, weather, and the fact you slept like a smashed crab. Your brain is the sensor that integrates the whole mess. The best calculator is the one between your ears.
What I want athletes to do is build a translation layer between these metrics. Learn what your key paces usually look like in heart rate, power, and RPE, so you can swap instruments when the terrain or the day demands it. You’re basically building your own internal conversion chart, so you can hold the correct intensity whether you’re on the road, on trails, in heat, in wind, or just having one of those days.
Here’s what that might look like. Say your long slow distance pace on flat ground is around 5:15 per km. Over time you notice that, on a normal day, that sits around 155 bpm, about 210 watts, and feels like roughly a 5 out of 10, comfortable enough to talk, steady enough to keep rolling. Once you know that, you can take the same effort onto trails where pace becomes useless. On a hilly singletrack loop you might be running 6:30 per km and still be sitting in the same 5 out of 10 effort band, because the terrain tax is higher. The goal is not to worship one number. The goal is to hit the right effort for the session, using whatever data behaves best in that moment, with RPE as the final reality check.
