The difference between going for the occasional run and training for a marathon is that while training for a marathon, runs can't be skipped. When it's cold and gross out, it takes an enormous amount of willpower to get outside and get the job done for the day. Sometimes I lack that willpower, but usually I make it. Despite the crap weather today, I made five miles happen. If anything, I'd like to think I'm building character by submitting myself to such conditions.
I've noticed, though, that some of the winter teams at my school use the hallways for their workouts. Sometimes I leave my classroom in the afternoon only to be bowled over by the track team doing windsprints.
During my two seasons of winter track in high school, I recall doing exactly one workout in the hallways, and that was when there was a foot of snow outside.
Running in the cold darkness after a long day of work is hardly enviable, but it does help me focus. In the short term, I have to focus on the steps directly in front of me, lest I trip over a root or crack in the sidewalk and plant my face on the ground. In the long term, the extreme discomfort helps me focus on the payoff that I envision down the road when I'm running the Boston Marathon.