I agree with a lot of other people in this thread that attempting to lose muscle mass is not a good idea. However—especially since you like distance riding—here's something that did cause me to lose muscle mass: I rode the Southern Tier and didn't eat nearly enough for weeks on end. After a certain point, the human body resorts to autophagy. My body was wasted at the end—arms, legs, torso. I had trouble lifting my bike over my head at the end for the ceremonial ocean photo.
I didn't weigh in at the start or finish, but I would guesstimate that my weight went from 160-165 lb at the start to 135 lb at the end. I don't recommend this.