I'm assuming you have fit taken care of, a saddle that works for you, gloves, cycling shorts... all should be to the point where 30 miles poses no comfort issues. Given that, it's preparation, pacing, eating, hydration.
Preparation
There are tons of resources online with training schedules, etc. Here are my two basic guidelines: 1) Don't increase your single ride distance more than 50%, and 2) Don't try to ride more in a day than you train in a week. Plenty of people have ignored these and been fine (including me). But I think they're good guidelines.
Pacing
Avoid the temptation to start out with a group that's too fast for you. You'll pay later and it will be harsh. Particularly in the first third when you might be tempted, ride well within your abilities. Later on it won't be so much of a problem to restrain yourself
Eating
Eat constantly. Eat before you start, eat while you ride, eat when you stop to rest, eat while you're crossing the finish line. When you see anyone else get something to eat, do the same. Bananas, fig bars, bagels and honey, trail mix. Again there are tons of resources online with cycling fuel advice. A good guideline is to eat the food you trained on. I suppose it's possible to eat too much, but I don't think I've ever eaten enough.
Hydration
I've heard of people over-hydrating, even to the point of being fatal. My experience is I rarely drink enough. Drink consistently; use one water bottle an hour as a starting point, and use your training to find what your body needs. You want to drink something that replaces electrolytes. Again a good guideline is to drink what you trained with.
In your training between now and the ride, work to find food and drinks that work for you.