Longest slope I have done was Ventoux. There are some steep parts to it but the average is only 7.5% for 21 Kms. I used a camelback with the energy drink and a bottle with water. Main problem I had was Heat and sweat loss. Sweat running into the eyes so took a bandana with me to stop that.

21kms and it was interesting- Good job I had the I-pod with me though as it could have been boring. And do not forget the Wind proof for the trip down.