I think I know the secret.
Now, I'm just gonna really simplify it and provide no explanation cos I'm lazy.
Everytime you go up the stairs, run.
Everytime.
Run at different paces, go at 2 steps at a time or whatever, but just do it.
I ran up the stairs my whole life, and my diet has been generally bad.
There's reasons behind it, but I'm lazy.
Trust me.