Neil Peart, when describing long-distance cycling, says it reminds him of a man who beats his head against the wall, and when his doctor asks him why he does it, he says it's because it feels so good when he stops.
There are moments of true joy in running, but (in my limited experience) none of them beat the moment when your run is complete. You did everything you set out to do, your heart finally begins to slow down, your legs understand that you're done abusing them, and the incessant "thump thump thump" noise of your feet hitting the road finally stops. You realize that the thing you were half-dreading is finally over, and you are filled with satisfaction and empowerment.
Some days it feels great to run, some days it feels awful, but it always feels awesome when I stop.
Of course, that only works if you start. It's hard to remind your body of that when you're trying to psych yourself into lacing up and heading out the door....
Update: 3.5 miles during lunch today, longest run of the year for me. Heart rate was higher than I'd like. Splurged a bit on sweets around Halloween but nothing disastrous; just enough to keep my weight hovering around the same number for a week now.