You don't even have to use weights, see how many full body weight squats you can do in a row (full ones, back straight and going right down). It gets extremely intense, especially the first time when the lactic acid starts kicking in. My aim is to get to 500 in a row, right now im up to about 230 and one of the most dreaded parts of my workouts. You shouldn't rely on one exercise to condition a part of your body, use a variety to gain the most benefit cardio and the legs are no different.