"If you can't hold a conversation while running, you're going too fast." That was the best advice I have heard, and I am using it on every run.
I am not sure if you knew this, but for beginners the trick is simple: run and chat. I did not realise that running at this pace puts your body into a proper aerobic state, so you are training more effectively than if you just ran hard. I am not a doctor and I cannot explain it in a technical way, but here is the simple version for anyone preparing for a marathon: run and have a chat on the phone. :)