It would help to know what you are eating/drinking before, during, and post ride now.
If your workout is in the morning, I'd assume you aren't eating before, and your last meal was the night before. You're running on empty. If you aren't already, get something in your system.. nutrigrain or fig bars, banana, something before you start, and 300 calories per hour thereafter while on the bike.
Another thing to consider. Bonking after a workout sounds suspiciously like a blood-sugar crash... quickly absorbed carbs or coffee jack up your blood sugar, insulin kicks in to squirrel it away for later, you feel like crap. On an empty stomach, the effect is even more pronounced. Usually, your body kind of tempers the insulin response during and after a workout, but every body's different. Ya gotta deal with what ya got.