I used to do cardio 3x/ week on the days I wasn't lifting. For years I did this. While I stayed toned and lean, I was not growing muscle or getting the definition I wanted. I good friend told me that I was "cardioing" too much. It took about 6 months, but I gradually reduced cardio until I figured out what works for me. I currently have NO cardio days. My muscles have grown and developed substantially since them. I no longer feel "skinny fit". Cardio health is still important. While I don't go out of my way to incorporate "cardio days" I do bike to and from the gym (10 minute ride) as a warm-up and cool down. I also bike to and from work 5 days a week (20 min ride). These rides are leisurely and enjoyable, not to mention easier on the environment. I trickle in an occasional 10 minute HIIT session on the stationary bike at my gym about once or twice a month just to keep my cardiovascular health up.