i think the word "cardio" is tossed around too much.
a 300lb heffer getting on the tread mill and walking for 10 min does not constitute cardio in my opinion. That is a warm up or a cool down.
someone like myself that does explosion intervals throughout a 30-45 min time frame would actually be doing something cardio wise.
I also think when marathon runners train by upping their mileage each week in preperation of a marathon would also be considered cardio. But a steady paced jog for that period of time would be considered high intensity.
i don't think anything low intensity works. Your endurance fractionally improves from walking or doing something like low intensity aerobics. Doing some of my "cardio" routines will show signs of improvement in your cardiovascular endurance in as little as two weeks.