Hi, I'm Grace. I'm a Senior New Product Development Technologist at Science in Sport, and I run.

Let's start with the good news.

Fuelling is a precise science. Precise, but learnable. And the more you practise it, the better you get.

So in this episode: what actually happens to your carb stores during a session. How to match your fuel to the session in front of you. And the mistakes that quietly cost runners time.

By the end, you'll feel a lot more confident about what to take, and when.

Glycogen, your body's carb store, is your main fuel source for marathon training. How quickly you get through it depends largely on the intensity and the duration of your session.

For performance, poor glycogen availability is a key limiting factor. As your carb stores decline, it gets harder to sustain the pace or the intensity you're aiming for. Exactly when that happens varies from runner to runner, and from session to session.

But you don't have to wait until your stores are running low to address this. Taking on carbs during exercise maintains energy availability, supports performance, and helps you hold the quality of longer or more demanding sessions.

Every run needs a specific fuel plan. Your sharp 45-minute session and your steady Sunday 20-miler place entirely different demands on your body.

So you assess your session for duration and intensity. You make your fuelling plan. Then you execute it.

Once you get to grips with that, fuelling stops feeling like guesswork. With practice — and that's important — it becomes an asset. Controllable. Reliable. Repeatable.

Let's start with your shorter sessions. Threshold runs, progression runs, intervals — anything between an hour and 90 minutes. (Each of those runs is explained in episode three.)

Your goal's straightforward. Keep your carb availability high enough to hold the quality of your work.

Here we're assuming you've started with carbs on board — from a normal, light, carb-rich meal two or three hours earlier. This assumption matters, because every following number depends on it.

For these sessions, you'll need between 40 and 60 grams of carbs per hour.

That's single-source territory. Single-source means one carbohydrate type, typically maltodextrin or glucose, that's absorbed quickly and utilised to provide energy. Simple, predictable, and perfect for these sessions.

That's exactly what our GO range was designed for — a GO Energy gel or drink does the job. Think of it as your reliable training partner.

Once your runs start to extend past an hour and a half, the picture changes. Your glycogen stores are faced with increased demand.

So our intake has to keep up with our work rate. Here you're looking at somewhere in the range of 60 to 90 grams an hour.

This is where single-source hits a limit. Maltodextrin — or glucose — is absorbed through one route in your gut, and that route has a ceiling. Push more at it and the excess just sits there, and that's where the discomfort comes from.

Our dual-source products combine maltodextrin and fructose, which are absorbed through different routes. That allows higher carb intakes to be taken in and used, minimising gut problems at those higher rates.

That's the science BETA Fuel was built on, and it still leads the way — including BETA Fuel 80 when you need the higher end.

But getting your fuel choice right is no use if you leave it 'til race day. Your training runs — especially the longer ones — are where you develop your on-the-move fuelling. Experiment. Learn what works for you:

• Amounts. Find the hourly intake you tolerate well, then build from there gradually.
• Formats. Drinks, gels, and chews. Try them all.
• Timing. Get familiar with your routine until it's instinctive.
• Flavours. Mix it up for variety, or stick with your favourites.

By the time race day arrives, nothing should feel new. And remember: your gut needs training just as much as your legs and your engine.

What about the mistakes that hold runners back?

We've all been guilty of not thinking our run through and not planning our fuelling — just getting out there and grabbing something when we can. Be accurate.

Then there's racing on a fuel plan you've never rehearsed at race intensity. Untested is unknown.

Another one? Thinking more carbs are always better. Only if that matches the work you're doing — and, even then, only when your system can handle them.

And copying another runner. It's your session, your duration, your pace, and your tolerance. So build your own fuelling plan.

Match your fuel plan to your session. Develop your individual strategy. Then rehearse it until it's second nature.

If SiS GO Fuel Energy is your everyday partner, SiS BETA Fuel is your long-run specialist — and your go-to marathon race-day option.

We were all a bit clueless about marathon fuelling at one stage. Even elite runners had to start somewhere.

Because when fuelling becomes something you're comfortable with, the only way is up. Or, in the case of your PB, down.

On your next run, make a precise plan for quality, volume, and timing, and deliver it.

Just by doing that, you're another step closer to your fastest marathon yet.