Projected personal bests?

After you've been running for a while, you have to wonder: "What is my best possible personal best likely to be"? Now obviously "best possible" is a pretty slippery idea, but I think you can define it something like this: If I trained smart and hard, didn't get slowed down by injury or sickness, was feeling great the day of, and pushed myself to my limits, what would my time be with better than 10% likelihood?

Here are some guesses:

5k: 20:30
10k: 45:00
21k: 1:43:00
42k: 3:50:00