I have found that a portable heart rate monitor has been an invaluable tool for introducing and increasing exercise. Mine has a wristband and chest band (Polar), but there are also just wristband ones.

Before I had the monitor, I could never really figure out why some walks would bring on the symptoms while others wouldn't. You use it to determine what you can tolerate without symptoms, and then ever so slowly begin increasing the heart rate. I did it in three week increments.