If you run regularly there'll be nice days. The temperature will be 55F with no breeze and cloudy enough so the sun isn't annoyingly bright. You'll feel great the whole time and the only regret will be that it wasn't a longer run.

Those aren't the days that matter.

The days that matter are rainy. They'll be bastard hot and humid. Cold and windy. You'll be annoyed because you don't have time. Something will hurt and there'll be a thought in the back of your head that maybe if you skip today (and the next run too?) then you'll feel better.

Those crap days are the days that count. Those days are money in the bank. Enough of them and you get great days. Every day like that is a day where you can think that running for you is like a smoke to a pack a day man. It's not something you do it's something you are.