I Am A 70 Y/o Male In Fairly Good Health

I am a 70 y/o male in fairly good health for my age. I have experienced a slight to sometimes significant cough (depending on level of exercise) with some mucous for about 10 years or so. It can be very annoying at times. I was a moderate smoker as a younger man, but have not for about 25 years. I am a mouth and belly breather so I don't do well on breathing tests, but I still exercise significantly and most of the exercise I do involves walking, often in the mountains. What might be going on here?