Since leaving college, she'd stayed in shape through the local gym, where she lifted weights and forced herself onto a cardio machine twice a week.
While most people either do strength training or cardio training, and some people switch days where one day they do strength and the next time they do cardio, a certain type of cardio workout combines both types of workout into one.
Others can't make that kind of time commitment, or perhaps they live in climates that don't lend themselves to the outdoors nature of a traditional Cardio Boot Camp; in these cases, modified indoor versions can fill the void nicely.
If you are going to put five three-minute songs in a row for a good cardio stint, you'll want to choose three songs with a fairly high number of beats per minute, but not so high that you can't sustain the level for fifteen minutes.
Other options for cardio/aerobic workouts are cardio spinning, swimming laps, speed walking, biking, cross-country skiing/snowshoeing, playing tennis/basketball/soccer...even playing a game of tag with your kids is aerobic exercise.