I know this response might be too late - but better than never!

"Studies have shown..." (meaning my coach has harped on me enough for this to sink in) that you should be drinking a minimum of 1 water bottle per hour. When you do take a sip, swig a quarter of the bottle at a time. This will keep you energized throughout the ride. Of course, the more time you ride the more calories you burn. You could geek out like me, and get really scientific by calculating exactly how much you're burning based on your heart rate and power, etc, etc . This will help you figure out exactly how many calories per hour you need to put back into your body during the ride. Needless to say, GU, Gatorade and all those other wonderful energy products should be stuffed in your pockets and consumed before bonking.

I have gotten in the bad habit of waiting till I feel slightly sluggish before downing a Gu. Energy bars typically take 15 minutes to feel their effects and had I eatten one 15 minutes ago - no bonk. Not ever. If you have a heart rate monitor with alerts - then perhaps you could set a reminder during the ride? I just might try that myself!

Another aspect to consider is your prep time. Did you sleep a minimum of 8 hours a night the week before your ride? Have you been pounding water the entire week? What is your stress factor? Did you eat what you normally eat before training rides?

I hope this helps - I could go on for a while on this subject.....