Good for you! I'm not a girl but I have trained in boxing and kickboxing for years, and I can say it is the best cardio and muscle toning workout you will ever have. Basically what it is, endurance training for your muscles.
The conditioning that you get from boxing is hard to beat. I think my shoulders have actually increased in size, too. Grappling gives boxing a run for it's money in the conditioning department, though... the workout I get training in Brazilian Jiu-itsu is unreal.
What style of kickboxing do you practice? I'm a Muay Thai guy.