its literally burn more calories than you consume = lose weight. you can either eat less (which to a degree can become unhealthy and will prevent the burning of fat) and/or burn more calories (it doesn't matter how you burn them, some activites burn calories faster than others).
running is one of the faster ways to burn calories (per unit time)
also, dont be a sedentary athlete:
http://www.beginnertriathlete.com/cm...articleid=1871