Write It Down and Schedule It!
After you've chosen a 10K in which to participate and mapped out what type of training routine you'll follow, make a chart. Then, record your progress; it'll not only provide motivation as you improve, but also let you know when you're truly ready to compete. You'll be less likely to cheat if you have to write down your daily distances in black and white. After all, who'd rather put a big zero on their progress chart than an actual distance?
Another way to use the power of the pen (or your electronic calendar) to help with motivation is to schedule your training sessions just like you would an appointment with someone you really want to spend time with. Train well and that person will soon be you -- in the mirror!
Finally, consider writing about your training odyssey on a blog or on social networking sites like Twitter or Facebook. Once you commit publicly to a challenge as significant as a 10K, it'll be harder to back down from your goal.