Most people assume fatigue in a marathon comes from weak legs or a lack of toughness.
My first marathon taught me otherwise.
I didn’t fall apart because my legs quit. I faded because my heart rate climbed too early and stayed there. Around mile nine or ten, it was already higher than it should’ve been for that stage of the race. Once that happens, you’re no longer managing effort — you’re reacting to it.
That’s when the run-walk started.
Not because I didn’t want to run. Because my body wouldn’t let me hold the pace without digging a hole I wouldn’t climb out of later.
Heart Rate Isn’t Just “Effort”
Long-term endurance training actually shifts how your heart is regulated. Studies show that consistent aerobic training increases parasympathetic activity and lowers submaximal heart rates — meaning experienced athletes can sustain effort with less cardio strain. (Endurance training and heart rate adaptation)
During prolonged exercise like a marathon, your heart rate naturally rises even at a steady workload — a phenomenon known as cardiovascular drift. This means your cardiovascular system requires pacing and control more than raw effort. (Cardiovascular drift during endurance exercise)
Studies also show that marathon performance is strongly linked to how efficiently the body uses oxygen — not just how hard you can push. This is why a controlled heart rate often beats a red-lining start. (VO2max and marathon performance determinants)
The Takeaway
The lesson here wasn’t frustration. It was clarity.
If you want to run farther, you don’t train by constantly redlining. You train by letting your cardiovascular system adapt and by learning restraint.
Endurance isn’t about how hard you can push. It’s about how long you can stay in control.