If you are are working out for several hours you are relying on lipid oxidation for your energy, not glycogen. One thing that the ever-running ALA infomercials don't tell you is that in type 2 diabetics, the only humans in which ALA has ever been shown to have a benefit, ALA shifts substrate utilization away from fats toward carbs (1), obviously a bad thing if you are trying to conserve glycogen stores or burn fat. I would download that whole article referenced below and read it if you are a serious athlete considering ALA.
One way ALA improves glucose utilization is by burning it at the expense of fat. Glucose is partially oxidized by glycolysis before the end product, pyruvate, enters the citric acid cycle for complete burning. The enzyme pyruvate dehydrogenase, which converts pyruvate to acetyl coenzyme A which enters the citric acid cycle for complete oxidation, is defective in type 2 diabetes. This creates a roadblock in the glucose utilization pathway, resulting in elevated plasma glucose and insulin resistance. ALA in some way stimulates pyruvate dehydrogenase, allowing more glucose to be burned, in turn allowing more glucose to enter cells.
This explains part of ALA's antidiabetic action. The downside is you are burning more carbs and glycogen instead of fat. This shift in substrate utilization is a feature of several different antidiabetic drugs, including the popular Avandia. It has been suggested that this is at least partly responsible for the weight gain associated with Avandia. It may be the case with ALA as well.
(1) Diabetes Care 1999 Feb;22(2):280-7
alpha-Lipoic acid treatment decreases serum lactate and pyruvate concentrations and improves glucose effectiveness in lean and obese patients with type 2 diabetes.
Konrad T, Vicini P, Kusterer K, Hoflich A, Assadkhani A, Bohles HJ, Sewell A, Tritschler HJ, Cobelli C, Usadel KH