To keep your lungs strong year after year...
The concept is pretty much the same as keeping your muscles strong.
You have to stress them in a way, that requires more strength than they currently have.
Which sends a message to your brain saying:
Hey, we need to devote more resources to making this thing stronger, if we want to continue living.
See your body, just like you, is lazy.
And it only wants to do as much work as it thinks it needs to get by.
And if your daily life doesn't require the work on its own...
You need to manufacture it yourself.
By exercising at the gym for example. However...