5K training – Three weeks to a PB?

After my huge success in the 8K (have you heard? 40:53) I have decided to run a 5K in three weeks in an attempt to set a new 5K PB. I’m pretty sure that if I have a good race I can destroy my current PB of 27:35. I ran a 8:13 pace for the 8K and if all I did was that pace again I’d do 5K in 25:31. If I take just 10 seconds off that pace I’ll finish the 5K in 25:00. So my goal is to finish in under 25 minutes. According to the Runner’s World Race Time Calculator I should be able to finish in 24:50. Of course the same calculator says I should be able to run a sub 4 hour marathon so I’m not sure if I believe it or not!