Helen
Its a well complicated issue but 'instant' energy for a run is sugars (as Redhead says, good from fruit like banana, but also energy bars, chocolate!! etc. but not much needed). Triathletes take this in on the cycle/run during long races (Ironman is 8 hours + of exercise) but also from energy drinks (which I find better).
As you get fitter, esp. at first, some fat is burnt but you are building muscle: which weighs MORE: oh no!
To reduce body fat you need to exercise longer term (weeks). My weight yo-yos depending on how much I do but my wife assures me that I'm fittest when I do a long run each week for at least 4 weeks and I swim. Swimming is fantastic for getting in shape (all muscles tone up) but the long slow run burns up fat in the body more efficiently. When the body uses sugars (Carbs) and when it burns fat is the complicated bit. I've been reading about it for years but the experts seem to argue!
Any way: exercise more, don't diet, eat healthy stuff, go for a swim, good luck in the half, its lovely up there!
Hope thats useful.
Rainer