Lose Weight, Gain Speed
This table, based on changes in maximal aerobic capacity, provides a rough estimate of how much your race times will improve if you lose weight, as long as you have it to lose. If your BMI drops below 18.5, you're at risk of becoming weaker and slower. http://www.runnersworld.com/article/0,7120,s6-242-304--11903-0,00.html