Ultimately we need fuel to get speed, but as the weather cools and our motivation to hit the trail wanes, it is important to keep the winter bulge in check to perform at a high level. A runner weighing 73 kilograms has to generate 6.5 per cent more energy to run the same pace as a runner weighing 68 kilograms. Penny Comins looks at some ways to keep active and fight the flab.