Choose the Right Clothing
What works for a 6 to 7 hour century probably isn't enough for an all-day ride. It may be chilly when you start. Keep your knees covered until it's 60 degrees, since the knees have very poor circulation and are prone to injury otherwise. Knee warmers, arm warmers and a windbreaker are often all you'll need. You could also stuff a couple of layers of newspaper under your jersey as added insulation and then throw them away.
I often start with thin polypro glove liners under my cycling gloves and a thin polypro balaclava under my helmet. If it might rain, take a raincoat--riding for a few hours in light gear in the rain is tolerable, but riding all day is no fun without a good raincoat. I also carry a shower cap from a motel--a good emergency head cover in the rain!
Use your training rides as experiments. Pay attention to all of your equipment and clothing. If something doesn't meet the performance requirements for the ride (for example, wrong gearing) or is the least bit uncomfortable, experiment until you find the solution. What is a minor problem for a few hours could become a major pain on an all-day ride.
I once asked Lon Haldeman, who has set seven transcontinental records, what is an ultra ride. He told me that when he was 10 years old he rode his coaster brake bike five miles to the water tower in the next town and thought he was a long-distance cyclist. The next month he rode 10 miles to the next water tower. Have fun meeting the challenge of your next water tower!Search for a cycling event.