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I am a devout runner, but I also have a huge sweet tooth, and I have great respect for the late chef and food-lover Julia Child, who advocated moderate amounts of “real food” such as real butter, and exhorted people to enjoy their food. “Fat gives things flavor,” she reportedly said. Of course, people with specific health and food issues need to be mindful of those issues, and if you are on a weight-loss diet different rules apply, but this is a marathon training book, not a diet book. In fact, with all the energy you're expending, it's OK to indulge in pretty much anything you like, with a dash of common sense.
You don't need to make a habit out of eating a whole pint of ice cream daily, but in my experience most runners, especially marathoners, can splurge on weekends and it won't show up on the scale. Some of your favorite things may have little or no nutritional value, so keep those things to a minimum and don't neglect the genuinely healthy food. I tend to indulge in dessert and peanut butter. By contrast, one of my training partners, who shall remain anonymous, likes to indulge in a trip to KFC periodically for a bucket of chicken and biscuits.
Disclaimer: I am not a professional nutritionist, but it's ok to enjoy a drink or dessert. If you over-indulge, forgive yourself and go for a nice run the next day.