I'm a type II diabetic and used to do what markm did and not eat while riding. I now replenish myself constantly and have found that my energy level is up and I can ride faster and farther than ever before. Weekend befor last we rode 101 miles on Saturday and 80 on Sunday, but I could have gone much further either day. Last weekend we rode 45 miles Saturday and Sunday averaging over 18mph both days.
My point here is you need to watch what you do, but you also need to replenish what your burning or you won't improve. I might add that my A1C is usually around 6.5 and I really don't have to watch my diet anymore. I do try to eat healthy and when I do eat sweets it is not real icky sweet sweets.
I know everyone is different, but when you cycle for 2 hours or better or even less your body continues burning long after that.
So just keep on riding, because it does amazing things to our health.
Philli....