I think you're just unfit, which won't be news to you.
What happened to you has happened to me, albeit at higher levels of activity, when I have suddenly increased my training load by a substantial margin. You aren't overtrained - overtraining is the accumulated effect of failing to recover properly over a long period of time - you're tired, that's all.
Don't stop. Gradually building time on the bike will make a big difference to your fitness surprisingly quickly. But listen to your body and give yourself time to recover between rides. Fitness comes with exercise and recovery, not just exercise. Initially I'd suggest riding every other day, with a day off in between. After a month or so you'll find the rest days are less necessary and you can ride five or six days a week.
As far as pre-ride nutrition is concerened, just eat normally. Your rides so far aren't long enough to worry too much about on-bike nutrition, either, but as CFB says above, you might find it useful to nibble. Make sure you have water with you.