My two cents ... if you are a person who doesn't get the annual blood fasting tests through your doctor, meaning you don't know what your blood sugars are, then call your doctor and request the blood tests. You may be pre-diabetic, diabetic or insulin resistant, and bonking at mile 5 is your body telling you that something is wrong.

If you are ok health-wise, and I sure hope you are, then like someone said, get to the gym and do cardio on the elliptical, plus take spin class, plus get out and do some walking/hiking. Build up your cardio off-the-bike so that you can do 90 minutes easily on another activity.

Another idea is you may be riding in the heat of the day and you are simply heat sensitive.

And check your electrolytes like others mentioned.

The categories are one - fitness ability, two - blood sugars, and three - electrolytes. Your problem will be in one of the categories. Unless .... hate to mention the obvious, but make sure your brakes are not rubbing against the rims. Flip up the little lever if you need to do so.