I've never trained for a marathon...I ran some 15Ks and found that muscle is not lost, but dehydration and burning fat makes my muscle mass more defined. I was a swimmer---and noticed the same thing at the peak of my training.
You just have to keep up the protein consumption and weight training to counter any muscle catabolism.
"His voice rose under the black smoke before the burning wreckage of the island; and infected by that emotion, the other little boys began to shake and sob too. And in the middle of them....Ralph wept for the end of innocence, the darkness of man’s heart…”. -----William Golding