If you're going to join a pacing group, go and vet the pacer at the registration expo beforehand. Just chat him up--how many pacing groups has he led, how many times has he run this particular course--Honestly, he/she should be a little older--they know what they're doing! These folks can be great if they're good--at the end, when your brain is fried, all you have to do is think about keeping up with them. But I've also had flakey ones who went out too fast and fell out of the race---
Best of luck!