Do you wave?
It’s been proven in numerous academic studies that cyclists and runners who fail to return waves have shorter life spans, sadder weekends, and terrible taste in music.
That line in this WSJ article made me laugh.
Waving when running is a bit weird, as you’re passing within a meter or two of the other runner. I mutter “How’s it going?” and move on. A personal greeting.
On the bike, normally the other cyclist is on the opposite side of the road, so a wave or nod is more appropriate. Greeting at a distance.
When approaching a runner/cyclist from behind is trickier. Do I say something and it might come off as smug in a “Quake at my awesome speed fellow earthling”, or nothing which could be interpreted as “I’m so good I don’t even acknowledge you”. I don’t think I can win (if there’s something there to be won) so I still mutter “How’s it going” and keep the head down.