What he said. I've heard from guys before that doing cardio causes you to lose muscle. These are the guys with the guts hanging over their shorts lifting weights in the gym. They just don't want to get themselves out of breath.
If you are lifting and eating right, you won't lose muscle mass. You can do cardio 7 days a week, twice per day, and you'll feel great.
My personal preference is to mix up what I do for cardio, between running, swimming, biking, etc. But it's up to you. Pay attention to what your body is telling you. If you run every day for a couple of weeks and start to feel some pain in your joints, throw in some swimming and some biking.
There are so many cardio exercises that are enjoyable... just have fun with it!
