Remember your heart is a muscle. Working it out and making it beat faster due to exercise will only make it stronger. Not working it out will make the muscle weak and more susceptible to bad things. Also, the heart is a pump and the more you workout you oxygenate your vascular system thus increasing the diameter of your arteries. You should panic when you don't exercise your heart. And if your heart wasn't beating faster from workouts then you would really have a problem.
Also, many studies show that regular exercise increases blood flow to the brain which is a key component in overcoming anxiety and depression.
(I also used to trip out a little bit when I would play basketball and my heart rate would near 200 bpm. But I got over it quickly since it is perfectly natural.)
"If man made it, don't eat it" - Jack Lallane
People say I am on a "crazy" diet. What is so crazy about eating veggies, fruits, seafood and organ meats? Just because I don't eat whole wheat and processed food doesn't make my diet "crazy". Maybe everyone else with a SAD are the "crazy" ones for putting that junk in their system.