The long-run HR climb: cardiac drift over the marathon distance
The first time you run a heart-rate-capped long run, something maddening happens. You hold the exact same easy pace the whole way, but your heart rate keeps climbing — and by the end you're a full zone higher than where you started, at no extra effort. Nothing is wrong. That's cardiac drift over distance, and on long runs it's not a rounding error.
On a long effort it's normal to see heart rate drift upward by as much as 15% at an unchanged pace, depending on conditions. That's enough to bump you a whole training zone. Picture a 40-year-old running genuinely easy in Zone 2 — around 65% of max heart rate, call it 117 beats per minute. Two-plus hours in, at the identical pace and effort, that can drift to roughly 135 bpm. If you'd slavishly held the starting heart rate instead, you'd have had to walk by the end. This is exactly why pace-by-heart-rate breaks down the longer you go.
What's driving it is mostly fluid. As you sweat over a long run, plasma volume drops, so less blood returns to fill the heart each beat — and heart rate rises to compensate. Heat stress piles on by sending blood to the skin for cooling, raising overall demand. The encouraging part is that you have a lever. Hydration visibly blunts the climb: runners who took in no fluid showed roughly a 10% rise in heart rate, while runners who replaced fluid at their sweat rate held it closer to 5%. Drinking on the run isn't fussiness — it measurably flattens your drift.
The racing lesson follows directly. Because drift inflates your back-half heart rate no matter how fit you are, the runners who hold together over a marathon are the ones who pace evenly and fuel consistently from the start — not the ones chasing a fixed heart-rate number into the late miles. Treat a rising heart rate on the long run as expected information, manage it with even-pacing and a fueling plan, and the back half stops being where your race quietly ends.
RunNerd expects your heart rate to drift up on long runs and doesn't punish you for it. Rather than holding you to a fixed HR ceiling for the whole run, it interprets the back-half climb in light of duration, heat, and how well you fueled and hydrated. If your drift is large and you took in little fluid, the coach reads that as a fueling-and-pacing problem to fix, not a fitness failure — and it pushes even-pacing and a real drinking plan on your longest efforts so race day doesn't fall apart in the final miles.