Scskowron, you beat me to it. Agree 100% on the lactic acid. Lactic acid gets cycled back into the Krebs cycle where it is used for fuel. Most, if not all, is used up during exercise and any residual amount that may remain is cleared out within an hour.
As for the energy systems, they do indeed work together. There was a neat chart that showed the contributions of the energy systems in various activities. For example, a 400m runner uses something like 50% aerobic and 50% anaerobic - don’t quote me on this because I can’t remember the exact percentage, but that’s close. By contrast, a 100m sprinter uses 90+% anaerobic and a marathoner uses 90+% aerobic (there is a small percentage contributed by the anaerobic system but it is small). A lot of people have taken Tabata’s study and concluded that all you need is anaerobic training to improve aerobic capacity. This is not correct. Specificity rules, here. Although improving anaerobic endurance will improve aerobic endurance to a certain extent, it cannot replace long, slow-distance training if the goal is to improve endurance.
Pdub, it sounds like your exercise science program is using very outdated information. The lactic acid thing has been out for a few years. Scary that a hack like myself who dabbles in fitness knows more than Ph.D.s in the field.