I think you might need to look at this from a perspective of time spent doing a particular activity. The measure you need to think about is time spent doing a particular activity. The traditional approach to cardiovascular improvement is to perform an aerobic bout of exercise for a minimum of 30 minutes (For sake of argument, a 4 mile run at a 7:00 per mile pace). Because there is limited variance in the activity, you could experience muscle breakdown if this becomes your sole activity for training. When you begin to embrace the lactate threshold training that crossfit endorses, you will still improve your aerobic, but by doing so through intense and varied anaerobic activity.
Just a quick side note, I ran a marathon back in 2000. Afterwards, I became extremely fatiqued and literally could not run a step for months. That is when I really began playing with anaerobic training and power lifting. I later discovered crossfit. This past week at the invitation of a neighbor, I ran my first true long run. We went six miles in 48 minutes, and I felt fine. You can still run some distance, but I would suggest that you keep it to once week.