Fat burning is primarily post the energy wall. Its where the body switches energy source. Any sustained long exercise session will start burning fat. However fat requires more oxygen than glucose, putting more demand on the energy production if oxygen is limited, and so increasing lactic acid production. Again, its about which system dominates, we don't just switch from one system to another entirely. Burning fat for fuel does not require glycolysis. That is restricted to glucose (and possibly pyruvate and glycerol from other sources at the end) as a fuel source. Fats use a process called beta oxidation instead. Some models might imply that beta oxidation is compromised, which might mean that at high intensity we cannot properly fat for burn fuel, so we cannot switch to fat burning metabolism. There are other models of ME though. Fat feeds into the Krebs cycle, just as the glucose product pyruvate does, but the common substrate is acetyl coA. However amino acids go directly to the Krebs cycle and where depends on the amino. My memory of this stuff is now very bad, its been too long since I studied it.