I'm going with the assumption this is a supported bike ride and not a race. I can't imagine racing a distance where my goal is "making it the whole 40".
As has been mentioned I find it easier to prep for a supported ride than a training ride. In a training ride you have to take food and water. In a 40 mile ride with three sag stops, I'd probably take water and some sort of emergency food such as a banana or energy bar and just stop at one or two of the stops.
As far as eating I find eating I find my normal breakfast works fine, as long as it isn't a big meal I can eat and ride within half an hour and be fine (much better than running).
The one advice I haven't seen mentioned isn't about eating, but just pace yourself at the start.
If you did 30 comfortably you can do 40 it is all about pace.
Enjoy your ride and don't over think it.
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"I've been dropped a lot of times, but it's never been because of my bike." DXchulo