Still, the idea that you can improve running economy—a trait that’s notoriously difficult to improve by “fixing” your running form—simply by smiling is remarkable. The most likely explanation is that smiling primes a more relaxed emotional state, in turn reducing sympathetic nervous system activity (which can influence heart rate) and muscle tension, allowing you to run more efficiently. There’s lots more research needed to confirm that idea, but for now, it’s a pretty low-risk intervention to try. Maybe someone will even smile back.
— Smiling Makes You a More Efficient Runner

So that's Eliud Kipchoge’s secret. Apparently the improvement can be a little over 2% which means if I smile my way through my next marathon I could easily cruise under the 2:40 mark. No more serious race face for me.