I find the major challenge in my running is getting out the door at all. It's a real struggle for me sometimes to get out there after a sh*tty day at work, and there are MANY of them right now. It's just a matter of remembering how fantastic I feel both mentally and physically after a run. The person who could bottle that feeling would be rich indeed.
On the positive side, I'm much better at pushing myself and my limits during my actual run. Someone on another running board I follow has a quote in his siggy: "Most people never run far enough on their first wind to find out they've got a second. " - William James. I'm trying to live by that now - hey, my legs might be tired, my lungs might be about to burst on a tough hill, but darn it, I'm going to keep going - and sure enough, once I reach the top, I've completed it and I'm still moving. Hills, distance, speed, I'm trying to work that "second wind" on all of them, and am gratified to find it holds true more often than not.