If you’re training for a marathon you don’t go out and do the same run every day. You do some faster, some slower, some longer, some shorter, because that improves your fitness. What we see with a lot of people is that despite the fact they’re doing different training day to day, they still eat the same amount or the same foods every day.
For long duration runs, it’s really good to fuel up beforehand with a good amount of carbohydrates, and take on some fuel during it as well. If you’re running for more than 60 minutes we’d advise you to take on carbohydrates before—and take some carbohydrates with you. That can be from gels, drinks, sports foods or, if you’re OK with eating whole foods, flapjacks, rice cakes.
For shorter, higher-intensity runs, you don’t need to eat more, but aim to increase the percentage of carbohydrate-based foods on your plate. If a plate on a normal day is a third carbohydrate, a third protein and a third fruit and vegetables, shift that plate to maybe half carbohydrate and then a quarter protein and a quarter vegetables—so you’re eating the same amount of calories, but the proportion that’s coming from carbs is greater.
It’s about matching requirements. If you have a recovery run, or a shorter distance run, you don’t need to fuel specifically for those. Just eating some food is going to be a good thing before that, and that’s covered in most people’s day-to-day diet.