I did one of those events not too long ago - I ran the 10K easy and then ran the 5K as hard as I could. The biggest problem I found was the cool-down in between each race. So I wound up jogging around the area until the gun went off for the second race. The trick of course was not to wear myself out, but not cool down so as to let my legs get stiff before the 5K start. I took the event as a fun strategy run, i.e., what would it be like to do it? And yes, it was fun!
Good luck!
-bill