Durability: The Fourth Pillar of Marathon Performance

The real reason you blow up at mile 20 probably isn't your aerobic base — it's that your aerobic base falls apart under fatigue.

Researchers have started formalizing what coaches have suspected for years: VO2max, lactate threshold, and running economy don't hold steady as a race progresses. They drift — sometimes badly. The term gaining traction is durability (also called physiological resilience), and it's now being treated as a fourth, distinct determinant of marathon performance alongside the classic three.

What durability actually means

A durable runner is one whose physiological ceiling drops as little as possible over the course of a long effort. An undurable runner might have a great 5K VO2max, solid threshold pace, and efficient mechanics — but all three metrics erode meaningfully after 90+ minutes of running. That erosion shows up as pace drift, HR creep, and a very bad final 10K.

The mechanisms aren't fully nailed down, but candidates include glycogen depletion altering substrate use, neuromuscular fatigue changing motor-unit recruitment, and cellular stress accumulating in slow-twitch fibers over time. The result is that your body stops running like itself.

The trainable part

Here's the useful bit: durability responds to training. Specifically, three approaches show up in the literature and applied coaching:

1. Fatigued-state work — marathon-pace segments in the final miles of a long run, or a quality session performed after a prior hard day. You're teaching the body to hold form and intensity when it's already stressed.
2. Depletion runs — long runs done in a glycogen-reduced state to stress the metabolic machinery.
3. Heavy strength and plyometrics — resistance training improves the neuromuscular side of resilience, keeping stride mechanics cleaner late in races.

For your training

If your long runs are steady-effort slogs and your strength work is optional, you're probably building fitness without building durability. The two aren't the same. During build and peak phases, audit your long runs: when do marathon-pace miles appear? If the answer is "mostly in the first half," flip that.

Durability is boring to train. It's also probably why some runners with modest VO2max numbers run surprisingly fast marathons.