Elaine, if two days fits best, you've answered your own question! Since you do plenty of cardio, hitting each muscle group twice a week is great! MIS, which is a favorite of mine, is a perfect twice a week tape! You will not necessarily get better results by adding more strength training. My advice would be do it twice a week and add on if you don't like your results. I'll bet you'll be satisfied though!;-)
Bobbi