Cardio for one hour?

ok so i go to the gym..although i havent gone alll summer..im gonna go today! wooo. ok so one time when i went to the gym..they had a sign that read "Too much cardio increases fat". i dont really feel like doing any machines today..and im going with another girl who wants to stay at the gym for one hour. if i do 30 minutes on the bike and 30 minutes on the treadmill..is that ok? or will that "increase fat"