Injuries are our best teachers.
Train at your current fitness level, or slightly above — not where you want to be.
As powerful as our legs are, as magnificent as our lungs and arms and muscles are, nothing matters more than the mind.
Train where your fitness is NOW, not where you want to be.
It's a hard, simple calculus: Run until you can't run anymore. Then run some more. Find a new source of energy and will. Then run even faster.
What we eat is a matter of life and death.