Apparently, given time, and if you stop abusing it further, the lungs can repair itself.
Exercising can be a tedious affair. I used to jog about 6 km a day until the pain in my knees forced me to give up jogging. Now, I walk 5 km, sometimes 6 km a day. If I try to jog now, I will be out of breath after a few meters.
I have given up worrying about it. The thing is to just listen to your body and go with the flow. There will be days when you feel you could go a bit more, and there will be days when enough is enough.
Keeping fit is a long term commitment. No need to rush it. :-)