Muscle mass per some volume is about triple the mass of fat for the same volume, so JUST working your guns should increase mass, theoretically.. Although, obviously it's a better mass to have..
Riding an exercise bike is a fantastic way to lose weight.. Your muscles hardly get sore, compared to running or swimming or rowing etc, and you can go for as long as you like pretty much! I lost 16kg with cycling as my only form of exercise in the second half of last year. Also, if you have one at home you can just park it in front of your telly and watch whatever show you want or listen to whatever music, which makes it hardly like a chore.