It doesn't matter you do running or fast walking. The important thing is to keep your heart rate at aerobic exercise range. The aerobic exercise range is (220-your age)x 60% to 80%. If you are 30, the heart rate should be kept at 114 to 152. In this range, the fat will be consumed, the fat will burnt only after 40min. So, you have to continue the aerobic exercise more than 40min in oreder to really burn the fat. I am 170cm and 210lbs in late 2007 and recently I am 167lbs but there is still 20lbs to go. I do firstly cycling and swimming to reduce my weight. Running may be not good as it may hurt your knees due to your heavy weight.