Runner's Lows

Quite a lot has been written about a "Runner's High." The flip side is the lows a runner experiences when he/she cannot do the running he/she has become addicted to. I used to feel sick if I could not run a distance every day, but have slowly de-runified myself.
Another depressing feeling is when it becomes a struggle to run a distance which one could cover easily before. Ah, the anti-gravity of youth...