Cadence drift tells you the marathon is over before your legs do

The watch number that actually predicts a marathon blow-up is not heart rate or pace but the quiet slide in step rate.

Thirty kilometres in, the pace on the watch still looks fine. Heart rate looks fine, maybe a touch lower than it was, which the runner reads as a good sign and which is often the opposite. The number that has already turned is step rate, and it turned around twenty four kilometres, and nobody looked.

Cadence in a distance runner is not a target to hit. The famous figure of a hundred and eighty steps a minute came from a small observation of elite track runners and has been repeated ever since as though it were a law that applies to a ninety kilogram amateur running a five hour race. It does not. Everyone has their own comfortable range, set mostly by leg length and how much force they can put down.

What matters is the change. A runner holding steady pace with a step rate that has fallen four or five per cent from their first ten kilometres is taking longer strides at the same speed, which means longer flight and more braking on each landing, which means more eccentric load through the quadriceps at exactly the point where those muscles are already the limiting tissue. That is the mechanism behind the classic late collapse. Not the heart. The legs, taking a beating they were not taking at halfway, on tissue that has been absorbing shock for two and a half hours.

Cardiac drift gets all the attention because heart rate is easy to display. It is also the least actionable thing on the screen, since by the time it moves you are already in trouble and the only response is to slow down.

Step rate moves earlier. If a runner watches it and holds it deliberately by shortening the stride slightly and letting the pace go by a few seconds a kilometre, they usually finish having lost less time than they would have lost to the wall. That is a real tactical decision available at kilometre twenty five that most people never make, because their screen is showing them the wrong field.

I would set the watch to display cadence and elapsed time. Nothing else. Pace is a temptation and heart rate is a rearview mirror.

The one honest early warning you get, you get from your feet.