I don't know if having weight to loss would impact how quickly muscle mass builds. I've always been able to build it up pretty fast once I start working out.
Anyway: Tomorrow is going to be my first down day. I'll treat it as a 24 hour fast. From dinner tonight until the same time tomorrow night. Just some coffee and limited cream during the day. Not sure what to break the fast with, though. Or even if I should, given the coffee/cream consumption.