I've seen lots of people doing this, but never tried it myself. My cardio is pretty intense (for me) and not for long peroids of time. I wll go to a tennis court or the soccer field. On the court I will run around them (4 put together where I live) 10 times to get warmed up and then I'll proceed to running lines on one court, this is starting on the outside line and running across the court touching every line when I come to it and then running back before I go to the next one. I will take a few minutes break and then do the same thing, but with 2 courts. After this I take another few minute break and I'll run a different line drill from the baseline. On the soccer field I will jog along the sidelines and when I get to the end of the field I will sprint across to job the other side and sprint across the other end. I usually will run 10-20 of these. This seems to work really well for me, but I haven't been doing it much since I moved since I'm trying to bulk right now.