Hi breakitbuyit! My DS is a C2C on the team so I asked him for some info for you.
1) You don't worry about maintaining XC fitness during BCT, you just try to get through BCT. Coach realizes what you are going through. My slender DS gained 17 pounds of upper body muscle during BCT. These...
Last summer my son gained 15 pounds of muscle during BCT. It was mostly upper body muscle. He is a slender middle distance runner and got back to his running weight during the fall. There were several distance runners that gained weight during BCT.