Whenever I see a fellow runner I always try to make eye contact and either raise my hand to say hi, mutter a breathless 'hi' or give a quick nod of my head to acknowledge a fellow runner. I find this behavior is just a simple friendly gesture, that to me means 'I know what you're going through, and you're doing well', but not everyone behaves like this.
I've noticed the older generation of runners are always friendly, even managing a full breathed 'hi' whilst running, where as the younger generation don't seem to acknowledge a fellow runner.
You may laugh at that statement, I'm 27, so (I hope) you would class me in the younger generation, I don't know why other younger runners don't make eye contact. It's good to know you're not on your own, that there are plenty of others out there running down a hill you're trying to run up.
I may have head phones on, but I can still say 'Hi'...