For me it's the challenge to try to beat myself or do better than I did in the past. I try to keep in mind not what I have accomplished, but what I have to try to accomplish in the future.
Granted, I've only accomplished two half marathons, so I really don't have to dwell too much on 'what I've accomplished'. Life is about moving forward. It's about the goal, the finish, what lies ahead. During a race you don't look back and dwell on the steps you've taken, you aim for the steps you're about to take and the future accomplishment (the finish).
I think the same is true for training. You can't dwell on the training you've done up to this point, only the training yet to be accomplished. Right now my stress is the 6 miles I'm suppose to run today, but my focus should be on the future miles, the miles yet to be ran. Today's run will help give me the endurance needed for those future miles.