I'm with KSH and Wahine on this both. Lower your pace a bit right at the start. You have to have a pace you can maintain.

And food. I think the energy drink is not enough. Try to time your food and drink intake so it's pretty steady and you're taking it in before you feel like you need it.

You are obviously not a wimp, and I don't think this is a symptom of overtraining, I think it's just trying to do an endurance ride at a sprint pace.

Give it a try and let us know how it goes!

Hugs and butterflies,
~T~