I've done more than 10 solo centuries over the last couple of years. I take a debit card and stop if I need food/water/beverages.
Some of these rides were trying new routes or going for a long ride on a day others weren't available. It also gets very hot in the summer in Phoenix and I've put the bike in the truck and drove up to the mountains and ridden in the cooler weather.
I don't worry about speed when I'm on a solo century ride.
2 tubes, 3 CO2 canisters, multi-tool, tire levers, latex gloves, plenty of energy bars, some cash, and a back-up air pump are the supplies I take.