Well, everyone's different. I find if I don't eat something every 15 miles or so, I can get shaky and dizzy, and it comes on really fast. As my fitness improves maybe I'll need to eat less. In the meantime, for a 40-ish mile ride I'll generally carry about half a dozen fig newtons in a snack-size ziplock, a banana, a couple of packs of gu (I actually like Mocha Mocha Clif Shots), maybe a special K bar, and a couple of bottles of water. I might not eat all that but I'm glad I have it if I need it. The Cliff shots are especially nice near the end of the ride when I'm prone to get the shakes easily. I tried Cytomax drink once and it didn't appeal to me, but you might like it and find it effective. I also make sure to have some sort of breakfast about an hour before I set out, generally a big bowl of special K cereal.

I did a 55-mile ride this weekend and I had all that plus a bagel and cream cheese at a coffee shop en route, and I didn't get the shakes (my thighs were pretty tired at the end though, we rode into a headwind for the last 5 miles, eww).

hope this helps, - Jo.