Is that 40 miles in a single day, or have you observed similar results when on tour?

Keeping in mind that my comments in no way resemble medical advice it makes sense that your blood sugar levels will be low(er) after a few hours of sustained efforts. When you're engaged in an endurance sport, your body is burning through as much blood glucose as it can get.

I'd just make sure you stay in a normal blood sugar range; if it gets too low, you'll bonk. Apparently this can be more difficult for a diabetes patient, since your body/brain doesn't automatically regulate insulin production and can't compensate for low blood sugar.

A bit more info that may be helpful.... http://books.google.com/books?id=XMU...20bonk&f=false