Folks often said to me, "Oh, you're a teacher -- that's so great! You must love it! It must be so much FUN!" and I would look at them, force a smile, and decide if this person was worth an explanation on what teaching is really like.
Rigor is pushing yourself beyond what is easy. Sports, reading, math, anger management, dieting -- to make progress in anything there has to be an element of rigor. Plain and simple.