I'm reading a running/racewalking memoir by Lawrence Block. In the book, he said that in the beginning he had no intention of participating in Marathon. But then the idea entered his mind, and he noted:
"I appreciated the fact that running would take off weight... this alone seemed reason enough to put on my shorts and shoes and get out there. Still, the phrase echoed, and began to do its subtle damage. Not the promise so much as the premise. The one word, really. Marathon."
What does "Not the promise so much as the premise" mean?