If we don't panic at the first sign of difficulty, we can back off a notch and ride through it. A mile or two reveals whether it is serious, and when the rough spot passes, we will have lost little. Sometimes all that is necessary is an internal adjustment to the increasing difficulty as the miles add up. The marathon does not permit delusions past the middle miles. We may ignore the signs that we should adjust our goals and strategies, but we will pay for it later.
Some indicators are concrete and non-negotiable.
The temperature in the 1993 New York City Marathon climbed to the mid-70s before I reached halfway. A year earlier I had run in similar conditions in Pueblo, Colorado, ignored the early signs of dehydration, and suffered debilitating cramps as my reward. I spent the night before the 1982 Maine Coast Marathon drenched in a fever sweat. Even though on a PR pace at mile 18, I stepped off the course rather than face the last 8. A friend went into the 1997 Boston ignoring a knee pain that had plagued him for six weeks.