Holding your output steady across the whole course profile instead of pushing on climbs. In Ironman World Championship data, low heart-rate variation was the strongest marker of success.
Race data from the Ironman World Championship shows it plainly: athletes with barely any heart-rate variation between climbs and descents perform markedly better relative to their planned schedule — up to 6 to 14 percent faster on descents at minimal change in pulse. It isn't the strongest legs that win, it's the calmest profile. The typical mistake comes in two halves: attacking climbs full gas, and riding the first half hour after the swim exit too briskly because your pulse is still high and your sense of pace lies. The fix is a ceiling rather than a target — a wattage or heart rate you cross nowhere on the course, not on a climb and not into a headwind. What often gets missed: over sprint distance you can afford unevenness, the race is over before it bites. The longer the distance, the higher the interest — and it falls due on the run.