I'm a runner and running when your asthma is bad isn't a good thing. Before I knew I had asthma I did it all the time...I didn't understand that what I was doing was actually quite dangerous. My stamina did increase, but not proportional to the amount I trained...it was frustrating. I did interval training twice per week, but I was miserable because I had problems keeping a fast pace.
Victoireh is right. Exercise is very beneficial on many levels and should be done on a regular basis. Once you get your asthma under control, your running will improve tremendously and you'll feel way more comfortable while running, too. You do have to use some discretion when deciding whether or not to train. Like, if your sick (or getting over post-sickness asthma issues), coughing a lot or peak flow readings are low, it's not a good idea to run...at least that's my experience.
Happy Trails :)
Exercise induced asthma attacks are generally the first sign that asthma is not in control for most asthmatics. I would suggest you first see your doctor to review your treatment plan, before embarking on increasing your stamina to exercise.
However exercise is very beneficial and should be done regularly to help strengthen your muscles and lungs. When your asthma is in control, you will be amazed to see how much easier it is to exercise.