I have a spirometry test in an hour! Mine is because I am convinced that I no longer have asthma. My chest problems came about when I got a lot of chest infections, at that time I wasn’t really looking after myself properly, in the form of poor diet and drinking too much. I gave up drinking 4.5 years ago , got down to structured training again and have seen constant improvement since.
My fitness has plateaued a bit now, but that is to be expected as I can’t really see how I could go on improving infinitely. I’d expect you to see improvements as you get back into it, probably quite rapid at first and then leveling off as you reach a peak.
When I was diagnosed the consultant said my lungs had the capacity of a 70+ year old, I now haven’t had my inhalers for months (asthma nurse isn’t happy) raced TT’s last year and even won a little (very little) bit of prize money. So stick at it, it’ll be hard at first, put what you used to do out of your mind and take it one workout at a time. Good luck.