I hear it a lot. Most recently, at the last race I ran. "I've never seen a runner smile."
Now, I've seen photos of myself running, and I don't often smile. The look on my face would best be described as stupid. Usually, my mouth is open, jaw slack, eyes partly closed. At the end of the run, or particularly at the end of a race, my head goes back and my eyes close all the way. In the words of David Byrne, "People in ecstasy look stupid."
It's true, runners don't often smile during their runs. But people don't often smile during sex, either. Afterwards, yes.