Many people think that without use fat "turns into" muscle. It does not....EVER. However, muscle can atrophy and fat can be added to the body over time. In this case, eventually you WILL have less muscle and more fat, but I don't think I would go as far as to say a "bunch of flab in its place". Unless of course you eat nothing but Twinkies and Ho-Ho's!!! Actually, if it makes you feel better, Mars, I am pretty mesomorphic and weight training does little for my legs other than give me more strength and power. I am one of those people that can do nothing but cardio and have really muscular legs. So, there!! Especially if you are biking hard, especially mountain biking, you don't really have much to worry about if you are only taking the summer off. You will be sore when you resume weight training only because your body is no longer used to it, not so much because you have lost all your muscle..
Take Care,
Janice