Just in case you were thinking it was not possible to come back from what I was hit by 21 months ago, yesterday I completed week 9, the final week, of couch to 5k.
I'm not quite on 5k pace yet, but I managed 2.85 miles in 30 minutes, pretty good going for a 52 year old with lung scaring!
Better still, I was comfortably maintaining 160bpm HR while running, for 10 minutes, and the remaining 20 minutes were around 140-150bpm.
My threshold for aerobic to anaerobic is probably around 155-160bpm now!
21 months ago, just sitting up in bed had my HR around 130-140bpm and I was breathless. In fact its the detraining you suffer by been bedridden, plus the fluid and damage to lungs that causes the tachycardia.
So there you go, mission complete, with, 3 months to spare!
Now I just have to climb a bigger mountain than the small one I managed a few weeks ago!