The minor problem No. 1

Did I tell you I go running almost four times a week?
There is always this problem: You know when you are walking or running and someone comes to you from the opposite direction and you want to be nice and say hi or just give her/him a smile but you don't know when is the right time? It's been a problem for me since my childhood.
I just can't find the exact time to do this. Sometimes I don't look at all but it's mean and I don't like it. Sometimes I look at the person so early that she/he can't recognize my hi or smile. Sometimes, I do it so late when the person has already looked away!
It's a weird, I don't know how to deal with this situation.