You are definitely NOT bonking. Bonking is when your blood sugar drops really low, and with all that food ... that should not be happening!! Also bonking is a whole lot more serious than just feeling tired.
Besides the eating suggestions others have offered here (i.e. eat less), I'll make two suggestions:
1) Sleep - how much sleep are you getting each night? Perhaps you are just over tired.
2) Ride more - if 27.5 miles is your current limit, start gradually upping the distance on a couple days each week. After a month or two, when you're doing 40 and 50 and 60 mile rides, you'll come home after 27.5 miles and feel like you've hardly done anything.
