Sounds like you did exactly the right thing at the right time. Also sounds like you showed remarkable restraint in not pinching Mr. Full-Kit-Pace-Line's head off.

I ride trails one in a while and get moving, but I announce that I am "passing on the left" or "passing" or "passing on the right". When traffic is heavy with dogs, kids, strollers etc. I slow down to a crawl. Most of the time, people say thank you when I announce that I'm passing.

I've never seen pace lines on our trails, but I would say that if you're that much into it, get out on the road for that endeavor.