Fear: "I'm going to finish last."
Reality: Over the years, I've heard hundreds of beginner runners confess that coming in last place is their biggest fear when running their first race. The truth is that you probably have as much chance of finishing last as you do of finishing first. Most short-distance races (such as 5Ks) have a number of walkers participating, so if you're planning to run or run/walk, you're most likely going to finish ahead of a bunch of walkers. If you're really worried about finishing last, choose a short fun run or charity race that's walker-friendly with no time limit, so you know for certain that they'll be walkers in the race.
But, more importantly, try not to worry about how your performance compares to the other participants. Running in a road race is all about setting a goal and achieving it, and the only person you should be competing against is yourself. So whether you come in first or last, you're still accomplishing something major. And, at many races, spectators and race volunteers cheer loudly for the back-of-the-packers, so you may actually get more attention if you finish toward the end!