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.
I ask you to judge me by the enemies I have made. --FDR
I remember redwood trees, bumper cars and wolverines
The ocean's Trident submarines
Lemons, limes and tangerines
I remember this… R.E.M. 1988Cruel SummerMy Journal