The Science
Don't let this be you
The Bonk
Exhaustion, or "The Bonk," as it is lovingly referred to, is the point in which some physiological process in your body is not going quite correctly. Research and experience tells us "The Bonk," is usually due to one of these three factors; 1. Sugar 2. Hydration 3. Salt.
You likely need more than you think
Sugar
Modern research supports carbohydrate consumption after roughly an hour of exertion. We could talk for hours on osmolarity, sucrose to dextrose ratios, newer carbohydrates such as cyclic dextrin... but those are marginal gains. The most important aspect is amount! You should be aiming to consume 60-90 grams of carbohydrates per hour after initial hour of exertion. This is roughly 300 calories per hour!
You need more salt than you think
Hydration
Sodium is incredibly important for muscular contraction. You likely need more of it than you think. During ultra endurance events, a good rule of thumb is 500-700 mg of salt per hour (a typical gas station sports drink has likely 1/3 of this amount of sodium). Complement this with roughly 500 to 1,000 ml of fluid per hour depending on athlete, intensity, and heat.