Anyone who thinks bodybuilders or any athlete who carry a ton of mass can be proficient at running more than a few yards have lost their damn minds. Even if they trained for it, the body's natural tendency will be to strip mass in order to become more efficient at the task at hand. Therefore, they would become faster as their weight dropped, but by the time they became truly efficient, they would no longer be massive.
This is why bodybuilders don't run anything close to long distances, their body's would drop mass like there's no tomorrow.
Anyone who has become very heavy (whether it be muscle or fat) can attest to this.