Congratulations. It is a tough time to do a century with the heat and humidity. As Chasm said, you probably suffered from going to hard initially, eating too little and dehydration. It is generally a good idea to try to keep the pace down on a century until you reach about 70 miles. If you feel you have energy to burn then have at it. Otherwise the last few miles can be well....... challenging. But you kept at it. Way to go.