Here's a good quad-burner and cardio exercise for those with access to a gym - 20-30 minutes on an elliptical trainer, but every couple of minutes, do 60 seconds in a squat position. Just drop your ass, upper leg parallel to the floor at the highest position of each stroke, and let it burn. If you can maintain this position for much more than a minute at a time, you are a beast.
Sounds dumb, looks dumber, but really works.