SOMEDAY IS NOT A DAY OF THE WEEK.
As you build endurance, you also build your cardiovascular health and increase your lung capacity through a low impact workout (unlike running) which is safe for the knees.
Not only does a 45-60 minute indoor cycling class burn a serious amount of calories (up to 800), these workouts are good for the soul. As you tone your body, you also build mental strength. The added bonus, you are having so much fun pumping to the music you forget you are working out!