Well done on signing up for Relay for Life....it's such a great cause.
I'm doing the Moonwalk for Breast Cancer in May and it's the 3rd time....Also the 3rd time I have done it many stones overweight.
My best advice is take it at your own pace........It's your own acheivement as well as raising money for a good cause.
I know people that have done relay for life before and they always have a great time.
Make sure you enjoy it too