When you have asthma it is best to start out slowly, and take your time getting fit.
Swimming is recommended or, if you don’t have access to a pool, short burst types of exercise, like sprinting, are better than cross-country or long distance running.
As an adult asthmatic, take up sports that are mildly aerobic such as walking, golf, bowls and swimming.
And if you don't exercise and have musical talent, take up a wind instrument - flute, clarinet, hobo or trumpet. This will increase your lung capacity.