How much cardio for a six pack?

I'm 17, 5'8" and 123 lbs. I'm skinny and all, but I'm trying to get toned, not big, just leaner looking, more solid. Every day, I watch my diet and everything, and in the evening, I do about 20 minutes of rigorous exercise, upper body one day, lower body the next. It gets my heart pumping, It gets my entire body worked out and sweating. Since I learned to start alternating, I have tired myself out from indoor exercise, that I'm too exhausted to do my 4 mile walk. Which is why I am asking. If I can truly feel the indoor stuff I've been doing to the point of physical exhaustion, is that 4 mile walk really that important? My friend told me it wasn't, but I thought I'd get more advice.