Training is important because bad days happen. Bad runs happen. Slow runs happen. To decrease the chances that they'll happen during a race, you need to stack your odds. Run more.
I was undertrained for the 1/2 marathon in November. I survived, though.
I flat out DIDN'T train for this weekend's 10 Mile race. And I almost didn't survive. I certainly didn't put forth my best effort. It was a bad run. It would have been a so-so training run. But, sadly, it wasn't a training run.
Can you finish that 5k? Sure. But wouldn't it be better to ROCK that 5k? How do you ROCK a race? Training.