Glycogen Preservation & Threshold Running

Every runner in your group starts the race with a limited fuel tank. How fast that tank empties depends far more on how the first kilometres are run than most people think. As a pacer, protecting that fuel is one of your most important jobs, even though nobody can see it happening.

Two ways your body makes energy#

When you run, your muscles burn a mix of fat and carbohydrate.

• Fat is the big reserve. Even a lean runner carries enough for many hours of running. But fat is burned slowly, using plenty of oxygen, so it can only supply energy at an easy, comfortable effort. This is the aerobic ("with oxygen") path that dominates at a relaxed pace.
• Carbohydrate is stored in the muscles and liver as glycogen. It releases energy faster, so the body relies on it more and more as the effort rises. The store is small: a few hundred grams, often described as enough for roughly 90 minutes to two hours of hard running.

At an easy pace you burn a mix of both. As the pace gets harder, the mix shifts towards carbohydrate. Near your threshold (the hardest effort you could hold for about an hour, where breathing becomes laboured and talking is difficult), carbohydrate supplies most of the energy.

Why the first 5 km matter so much#

Carbohydrate use does not rise in a straight line with speed. It rises steeply as you get closer to threshold. That means a small overspeed costs far more fuel than it seems to.

Take the 4:00 marathon group with a target pace of 5:41/km. Excited runners and an adrenaline-filled start push the group to 5:31/km for the first 5 km, only 10 seconds per kilometre too fast.

The runners gain 50 seconds. But for a runner whose realistic limit is 4:00, the faster pace pushes them closer to threshold, where each kilometre takes a much bigger bite out of the glycogen store. The exact extra cost differs from person to person, so we do not put a single number on it. What is clear is the pattern: the extra fuel spent is out of proportion to the time gained, and it cannot be refilled during the race. Gels and drinks (around 60 g of carbohydrate per hour, see the Fuel planner) only slow the loss, they do not reverse it.

Those 50 seconds are easily lost many times over late in the race if a fuel-starved runner slows down sharply.

What this means for the pacer#

• Start controlled. Run the first 1–2 km slightly slower than target pace, as in our core training principles. Let the body settle in.
• Never bank time early. Seconds gained in the first 5 km are not savings; they are a loan with high interest.
• Explain it to your group. At the start, say it simply: "We start easy so you still have fuel at 35 km."

This is also why the Pacer Support Guide speaks of a conservative start in every strategy plan, and why it matters even more in heat and humidity.

Key takeaways#

• Fat is a large but slow energy source; glycogen is fast but limited to a few hundred grams.
• Carbohydrate use rises steeply as effort approaches threshold, so a small overspeed costs far more fuel than the time it gains.
• Glycogen spent early cannot be fully replaced during the race.
• A controlled start protects your runners' finish, not just your split times.