Whenever I pass someone going in the opposite direction on the running trail, I acknowledge them. I try to do this with everyone, no matter their pace. Even if they are walking.
How I acknowledge them is based how serious of a runner they are or if they acknowledge me first. I return any acknowledgement with the same acknowledgement. If someone is a very serious runner, I am likely to just give them a head nod. That means, “Yup, we’re both runners.” If they seem to be a little more casual, I will offer them a wave. Usually it’s just the raise of a hand on the side closest to them that means, “Hey, glad we’re both out here running.”
And then there’s a smile. This is a more friendly acknowledgment, but I find that my behavior is determined by societal norms. I would never smile at another dude. But I smile at almost all the women. Many smile back.