Hi, I'm Grace. When I'm not in the lab at Science in Sport, I'm out running — always looking to train better and race better, and here to help you do the same.

We all want a faster time on race day. So we train with structure, focus, and a plan.

Sometimes just "going for a run" is great. Lace up, set off, see what happens.

But marathon preparation needs a blend of clearly defined workouts. Duration, intensity, pace — each one is there for a reason.

Think of your training week as a toolkit. Each run is a different tool, with a different job. Together they develop the range of physiological adaptations you need for marathon performance.

Different sessions place different demands on your energy systems. As intensity and duration change, so does the balance of carbs and fat you burn — and your overall carb demand can vary a lot between sessions.

Which means your fuelling should always be precisely targeted to the demands of your run.

So — let's look at some of the most popular, or unpopular, sessions.

Your relaxed, steady, controlled run that builds your aerobic base without much fatigue. As long as you've eaten well and you're properly hydrated, you're good to go.

Where you gradually lift the pace through the run, holding your form as fatigue builds. Not much fun towards the end, but excellent for developing your resilience and finishing strength.

Start well fuelled, then take in 40 to 60 grams of carbs an hour. Longer progression runs push towards the upper end of that range. Because intensity increases through the session, carb use generally increases too — so fuel consistently from the start.

This is the ideal job for the SiS GO range. Gels or drinks are the most practical formats here.

Short, hard efforts with recoveries in between, to build speed and running economy. There's no hiding place here.

Unless you're out there a long time, single-source carbs are ideal for the 60 grams an hour you're looking for. Again, GO products are designed for just this purpose.

The sessions designed to help you sustain a challenging pace for longer, and they place a high demand on carbohydrate. Start with sufficient carb availability, then keep it topped up.

Base your fuel choice on duration. Under 90 minutes, you're in good hands with the GO Fuel range. Over 90, you need dual-source carbs — so SiS BETA Fuel is your go-to. (For more on single- and dual-source fuels, see episode two.)

The backbone of your whole marathon training block. The longer the session, the more carb availability decides the outcome. Neglect the intake and you limit the output — or, even worse, you grind to a halt.

You're looking at 60 to 90 grams an hour here. That's BETA Fuel territory.

The most important long run of all. This normally comes around three weeks before race day: a full dress rehearsal, with your whole routine, including your complete fuelling plan.

So your fuel choice for this one will be exactly what you're going to use on the day. Same products, same volume, same timing. Everything. Change one variable and you're testing a plan you're not going to run.

So we've looked at easy runs, progression runs, interval sessions, tempo and threshold work, long runs, and race practice.

Put them together in a joined-up programme, individual to you, and you haven't just got a collection of runs. You've got a system.

Each training run brings different demands and delivers different benefits. So each one calls for a different fuelling plan.

When you fuel each session for what it actually is, all your hard work out on the road pays off.

It's time to treat each session on its merits and match your fuelling precisely to its needs. Starting with your next run.

You'll feel the difference. And you'll be one step closer to your fastest marathon yet.