Energy for the Long Haul – How the Body Works Step by Step
When I head out for a long session, whether it’s on the bike or in my running shoes, my body has its own “plan.” It always feels easy at the start – my legs respond instantly, and every pedal stroke is powerful. That’s the muscle glycogen at work. The muscles store about 300–500 grams of glycogen, roughly 1,200–2,000 calories. It’s fast fuel that lets you maintain a high pace and tempo right away, but you have to remember that this reserve is limited. The harder you push, the faster it runs out.
After an hour, things are still flowing smoothly, but now the liver becomes increasingly important. It acts as the body’s regulator and holds about 100 grams of glycogen, equivalent to 400 calories. The liver releases glucose into the bloodstream, keeping blood sugar stable and ensuring the brain gets its energy. Without this balance, you’d quickly feel dizzy and lose focus – but thanks to the liver, you can keep the pressure on.
After two hours, it starts to feel heavier. The muscle glycogen stores are significantly depleted, and the body begins to switch to fat as fuel. There’s virtually unlimited energy here – even a lean person has tens of thousands of calories stored – but it’s slower fuel. This means you can’t maintain the same intensity, and it’s exactly why you might “hit the wall” when glycogen runs out. The body keeps going, but at a slower pace.
For me, long sessions have become a balancing act. I’ve learned to refuel at the right time: carbs the day before, a sports drink or banana along the way if the session lasts longer than two hours, and above all, not starting too hard.
I also know that easy long sessions train the body to use more fat as fuel, making me more endurance-efficient in the long run. If I forget to drink regularly, I immediately notice my energy crashing, long before the fuel is actually gone.
The conclusion is simple: the body is built for endurance. The muscles provide the explosive start, the liver keeps energy stable, and fat carries me further than I initially think is possible. By understanding how these reserves work and learning to manage them, I can push my limits much further – and enjoy the feeling of actually making it all the way.
