Types of Cardio

I'll be doing at least 30 minutes of moderate to high intensity cardio today. Since I really can't run well (it's really about lack of muscular endurance, esp in calves), I'm doing step ups on the incline step I bought from Target. There are also intervals where I exercise my calves with stepping up calf raises. It's difficult using this on the second floor of my house while trying to avoid complaint from below. Maybe a few weeks of this will get me better muscular endurance.