What's your diet like? You're not restricting carbs are you? If so, that may be part of the problem. Did you have a good breakfast before you went out?
Otherwise,you may be pushing yourself too much, trying to do too much too soon.
Whatever your time constrictions are see what distance you can comfortably do in that amount of time without feeling exhausted.
stay at that pace/mileage for a week. The next week increase your mileage by ten percent for the same amount of time.
And make sure your body has enough fuel for the trip.
There are lots of threads here that reference proper nutrition for distance riding.
Keep us posted, I'd like to hear how things improve for you!