Thanks for sharing your personal experience with Metformin. As you mentioned, the sudden drop in energy might well have been due to hypoglycemia. Particularly if those meals had been high-carb. Metformin has a stabilizing effect on blood sugar, which might have put the brakes on the rollercoaster of blood sugar that you experienced.

You mentioned your pre-diabetic blood sugar levels. If you refer to fasting glucose, and if you typically exercise in the morning before breakfast, then those elevated levels might simply be a sign of your body being conditioned to needing more sugar at that time. That's an observation I have made with some clients. But without knowing the details of your case I can't give you a satisfactory explanation. Certainly, all the other characterisitics that you describe (low resting pulse, high fitness level) are somehow counterintuitive to a diabetic scenario.