Have you done any research on training for a 1/2? According to runnersworld.com's running coach training software you should be running an 11 mile run on the week before your 1/2 (I'm doing a 1/2 in September). You could use your 5k as part of that 11 miles--just keep running after you it the finish line. LOL
Seriously, if you've never trained for a 1/2 check out runnersworld.com. The running coach training info is free.