I have been busting my butt lately to get in shape for some spring races that are coming up in about 5 weeks. One area I know needs improvement is my cardio system. I suspect that this is a consequence of lots of base miles over the last couple months to strengthen my legs but not too many intervals to work the heart and lungs yet. I am doing intervals twice a week now but am also taking two days per week off so I don't over do it and to allow plenty of recovery. But, I want to do more to make sure my lungs and heart are fit by race time. So, my question is, on my off days, would it be beneficial to do some running to strengthen my heart/lungs or am I going to overdo it and not allow proper recovery. I guess the "heart" of the question is, what is recovery for?...Just the skeletal muscles in the legs?... or do I need to allow my heart and lungs a day off to recover also?