"Treat it as if you are starting a new sport." One of the most important lines in the whole book, in my opinion. Page 40
So, put yourself in the shoes of someone who really is running for the first time. You started running just 3 months ago. You've got up to 20 mins at a steady pace, you ache a bit sometimes, you feel it the next morning! But you're pretty chuffed with how you're getting on, if truth be told.
So when I got carried away, distracted by club mates (!) last week, and ran for 45 minutes, I pressed stop on my watch with a bit of a grimace. And sure enough, I ached for two or three days afterwards.
Just like a beginner would if she or he were to do the same.
This really is like starting a new sport. But, like learning any new sport, it's fascinating and exciting!