I fasted 59 days last summer, with the supervision of a wonderful naturopath/nutritionist. At day 48 she had me switch to fresh fruit/veg juices for extra nutritional support.
I continued to do moderate exercise throughout the fast, with the exception of detox crises. Loss of muscle mass was never a protein, though my naturopath did have me add quality protein in the form of chicken breast and fresh fish to my cleansing/rebuilding diet post fast.
Muscle loss will never be a problem on a ten day cleanse! Fasts of this length are perfectly ok unsupervised. If you are concerned about any issues for a longer fast, you should consult a natural health professional who understands the process.