Just Poop More, Dummy
Yeah, this probably would have been my first guess if I weren't me. I poop every morning before I get any sort of workout on. I'm a man who relaxes on the toilet. I play some Trivia Crack, check Twitter, and update my fantasy teams. If there's still some crud in my colon, I might read part of a book, but trust me, I do not rush the process. I take care of business, clean up, and start stretching for my routine. Yet no matter how long I'm there, I will inevitably have to poop when I go on that run.
A Little Extra Boost
Obviously, when you have to poop, you're going to get some farts first. I use these farts to push myself forward on the run, as I can use my sphincter to take some stress off of my legs. It's basically like attaching a steam engine to my backside. It also encourages me to run faster, as I definitely don't want to linger around in that smell.
This is the most likely answer. As possibly the most evolved human ever, my body natually makes the optimal decisions for my benefit. You never know what might happen when you go out for a run. I could be chased by bears, lions, elephants, or birds. By getting rid of all of my waste, I become lighter and even faster.
So, there you have it, I'm just so physically evolved that my body is constantly in survival mode. "Fight or flight" works for some, but "Rumble in your pooper means you'll run super" is the way I live my life (Alternative life motto: "Drop some weight, delay Heaven's gate").