Eat drink as you would normally. You should not need energy gels to get you through a hm.
If you really are that worried then prepare yourself mentally. Know the pace at which you can run it. Don't attempt to run any faster, keep it comfortable. Just get used to the distance.
If time is not important then try to start mid pack, this will help prevent you from setting off too fast. Don't worry about what others around you are doing, go at your own pace.
relax and enjoy.