Hi, I'm Grace. I'm a Senior New Product Development Technologist at Science in Sport — and I run. I'm chasing my marathon PB, and if you are too, you're in the right place.

We've all had those runs where our energy just wasn't there. Either we never get going in the first place, or we're moving well and then everything becomes hard work.

And we've all had those days when we're flying. Our legs feel light, our breathing's relaxed, our energy's on tap.

The difference between the two? Fuelling.

So in this episode we're going to look at how you store and use energy. How to manage your carb intake around your different sessions. And what that does to the quality of your whole training block.

Our target? More effective training, and a better time on race day.

You store carbs as glycogen, in your muscles and liver. Think of it as your fuel tank.

Every run drains it. Harder and longer efforts deplete it faster. And across a marathon training block, the combination of long runs and high volume places a substantial, sustained demand on it.

So you've got three jobs. One, start your sessions with enough glycogen. Two, top it up accurately. Three, think of your fuelling across the whole week, not just by individual runs.

If you don't, your energy availability suffers. And that's where your performance starts to drop.

It's a simple principle. The harder and longer you run, the greater your demand for carbs. Your fuelling has to provide enough to support the intensity and the duration of the session in front of you.

For example, hitting the wall. It's not necessarily a fitness issue. It's fuelling.

We still see it in every single marathon. And it happens to well-trained runners. In most cases, it's a mismatch between the fuel the race demands and the carbs actually available.

Under-fuelling has real consequences. You struggle to hit your target pace. The quality of your work drops. The whole thing's just harder than it needs to be.

We've all done it. Something came up, you skipped lunch, you didn't get a chance to grab a snack. You didn't think you needed a sports drink. Maybe you'd never tried a gel. So you went with just water.

That's under-fuelling, right there. And consistent under-fuelling across your whole training block limits everything you're building.

But with more discipline and planning, getting it right is an easy win. And it's a win that makes a big difference — it improves every single run you do. On race day, those gains will be right there on the clock when you cross the finish line.

Now, not every session needs the same approach. Different runs place different demands on your energy systems. That's where you've got to be precise.

So let's get strategic about it. Four things to decide before you leave home.

• Type. What kind of session is it, and how long? A two-hour long run and a tempo run are different jobs, and they need different fuel. This is the decision everything else hangs off.
• Quality. What you take, and in what form. Drinks, gels and chews are your most practical options on the move. (Product mention slot: to be dropped in here.)
• Volume. Grams of carbs per hour, matched to the duration and intensity of your session. Be strict with your numbers.
• Timing. When you fuel. Before the session, and at planned intervals during it.

The specific strategies change depending on your session. We go into them in depth in the next two episodes.

But getting to your target numbers takes time.

Every marathon, we see runners who are clearly asking their stomach to handle fuel it's not ready for. And it's not pretty.

Your capacity to absorb fuel on the move isn't fixed. It adapts. The processes that move carbs from your gut into your bloodstream respond to regular exposure. Practise at race intensity, consistently, and your stomach gets better at handling volume.

So use your longer runs as rehearsals. Start early in your training block, not in the last few weeks. Build it up gradually, and you'll get more and more comfortable with higher carb numbers.

Then, based on how you feel in training, you can develop your race-day fuelling plan. One you know works.

It's all about good habits. You're looking for comfort, confidence, and consistency. And out on the road is where you'll find them.

Before long, correct fuelling becomes part of your regular routine — just like your warm-up, your pacing, and recording your session. And you arrive at race day with a plan you know works, because you've already run it.

Avoid consistent under-fuelling. Get strategic about type, quality, volume and timing. Practise on the move to train your gut. And make all this an automatic part of your routine.

Because good fuelling habits make a real difference. They improve the quality and consistency of all your training. You sustain your target pace. You finish key sessions feeling strong. You recover better. You adapt and improve.

And let's not forget the most important bit. You enjoy your running more.

So on your next long run, no more winging it — make an exact fuelling plan. Write it down. Execute it exactly.

Do that consistently, and it's a game-changer. Because it's on well-fuelled training runs that you take minutes off your PB.