When you look into the past what's the thing you missed the most?

The long nights with nothing to do but read; I'd prop my flashlight up against my left shoulder and read until the sun came back up. I don't remember what I ate or drank or if I even left my room. Days, months, and years blend together when you live like that. My whole life has seemed to blend together like that. You can't go back, can you, or change? I also miss running too and feeling the rhythem of my lungs and heart and the sound of my feet hitting the pavement like some sort of Melody.