We have a no puke rule. The purpose is performance, not puking.
Think chest/hips/ push, or CHP, when it's time for uphill running. Chest up, hips forward, push strongly off each foot.
Here is the start, there is the finish line. Between that, you have to run.
The most important force inside you for feeling better all the time is the will to get more fit.
The marathon is a competition between your will and your possibilities
The more you frame the marathon as a stressful experience, the more negative messages you'll receive. But it's just as easy to frame it as a positively challenging journey.