I'm certainly a fan of getting out there and trying to get a workout started, especially since you can often get through a whole workout as long as you can get through the first ten to 15 minutes. I would like to add a caveat to the discussion of "wimping out," however.
"Wimping out" is sometimes the smartest thing you can do. There are times when I wish the athletes I work with would listen more closely to their bodies and call it a day when they're not responding to their workouts. Here are a few good ways to tell if turning around is the right thing to do:
1. Your heart rate/power output doesn't match your perceived exertion
Though perceived exertion is often dismissed as being "unscientific," I believe it has to be taken into consideration when talking about training. You know how you normally feel when you're riding at a certain pace or intensity. If it takes a superhuman effort to raise your heart rate or power output to your normal cruising level, or to the level necessary for intervals, it's a sign that something's not right. Usually, it means you're fatigued and that you're better off doing a recovery ride or taking the day off. If this scenario continues for more than a few days, then it's very important to talk with your coach.
2. You feel like taking a nap by the side of the road or trail
Most days, you're raring to go when you head out for a workout, but then there are days when you just can't seem to get excited about training. You're yawning at stop lights and daydreaming instead of focusing on your workout. On days like these, it's important to get out there and see what happens after about 15 minutes. If you snap out of your funk and get your head in the game, then continue with the workout. If you can't get your head into it, turn around and go home. I'd rather see you miss that one workout and come back excited the next day instead of absentmindedly going through the motions for several workouts in a row.
And since this is the holiday season, there's one other piece of advice (it's not completely pertinent to the original post in this thread, but it's useful anyway): A shortened workout is better than no workout at all. So, if a holiday party or poor winter weather cuts your available time down to just 30 minutes, take it. It helps you stay in a consistent training routine and ensures that you're not slipping backwards in your fitness progression. Shortened workouts may not move you forward as much as complete ones do, but they'll help you keep the fitness you've worked so hard to build.