If you have asthma you should know pursed lip breathing and diaphragmatic breathing already. What are your triggers if not exercise? Yoga is great and so are deep breathing exercises. take a deep breath into your stomach, hold it and exhale thru pursed lips. Your problem is airway restriction on exhalation. The best thing is to stop the restriction with regular constant use of non systemic corticosteroids, Flovent, and a bronchodilator when you start to get tight. If you can't exercise you will never get the stamina you are looking for with bigger lungs. Management is the main thing, consistant management. Best of luck.