Couple of quotes from the NYTimes story..
Those who run in packs are part of a select society, or maybe a self-selected society. Anyone can join, but you have to run and you have to go to the designated meeting place at the designated time. You might join a club that sponsors runs each week or you might go to a place like a parking lot behind a school where runners gather on weekend mornings. It’s not hard to find these meeting places; local running groups and running stores know where to go. And when you show up, ready to run, the society opens up to you.*Registraion (free) required.
You may hear comments from passing motorists — one guy called out to me and my running friend Claire Brown, 34, “Bet you can’t catch me.” Out of his earshot, I muttered, “I’ll bet we could if he got out of his car.”