Don't fight nature. Don't race when tired or sore. If you're that way before the start, think what the hard kilometers are going to do later. When you're racing hard, you can't train hard. There's no substitute for racing as a sharpener.
Long runs are an important step to racing goals.
Too much of something leads to too much of nothing. Top results are only reached through pain. An Olympic Gold Medal is rarely won through talent alone. The next time you run out of GAS, fill up your tank with rest.