To run faster you need to be leaner. Just look at the world beating Kenyans, they are razor thin. Off cause you dont need to be that thin to achieve your running goals.
To understand why a leaner person is faster than overweight person look at this example.
Lets compare an a Subaru and an Audi, The Audi has more horsepower (335), but that horsepower has to pull a bigger mass (4,300 pounds). The Subaru has a little less horsepower (300), but since it has to move less weight (3,300 pounds), it’s much more nimble and can accelerate like a bullet.
When you run the biggest load is your body weight. The heavier you are the more fuel you need, and the more water you need. The lungs and heart of a heavy person will need to work twice as hard as of a lean person to do similar work.
So, if you want to improve on your running times, cut on those extra pounds. The more weight you lose the faster you can run.