Here is a simple leg exercise you can do even at work in your spare time. Warning, your co-workers might give you some funny looks.
Simply back up to a chair. Raise your left leg and put the top of your foot arch down on the seat. Standing on your right foot with it about 2 feet out from the edge of the chair your knee cap should be over the center of your foot when looking straight down. Use your body weight to do a simple squat not letting your quads go deeper than horizontal from the floor. I clasp my hands together behind my lower back and start my reps. You might have to make a few slight adjustments in your stance to feel comfortable.
The nice thing about this is, no equipment needed. Your body weight will provide all the resistance needed and you will improve your balance which is also important. Start out with 30 reps on the right leg, then switch over to the left leg. Soon you will be up to 50 reps times two for each leg. Also notice how your heart rate responds! This is my secret, and now it's yours.