What effect would an extra gym session a day have on muscle gain?

Since I'm not due to start my new job until the start if next month, I've got a lot of free time on my hands. It occurred to me: There's no reason I couldn't go to the gym an extra time each day, but my concern is this: What effect will this have on my muscle mass? Would too much exercise result in energy being taken from muscles as well as fat or would it simply cause the fat to be turned into muscle? According to my scales, I'm about 30% fat, so there's no shortage of fat to turn into muscle.