I did some research on pubmed regarding what "base building" does to you. There's much more going on, like import proteins rising in muscle cells / mitochondria,and the liver producing more apolipoprotein (carrier protein for fatty acids) etc, etc.

I am confused about lactate being used as fuel.
I was taught lactic acid is not very efficient as fuel in the muscle during exercise. The way I read the abstract (quoted below) is that some is used as fuel, but also lactate is only removed within the cells during recovery, when they have plenty of oxygen - but this does not produce a lot of energy compared to glycolysis (aerobic metabolism produces 19x more energy than anaerobic per mole of glucose).
There is a cycle in the liver to convert lactate back to glucose (which, however, requires energy).

Brooks GA. The lactate shuttle during exercise and recovery. Med Sci Sports Exerc. 1986 Jun;18(3):360-8. Review
Most (75%+) of the lactate formed during sustained, steady-rate exercise is removed by oxidation during exercise, and only a minor fraction (approximately 20%) is converted to glucose. Significant lactate extraction occurs during net lactate release from active skeletal muscle; the total lactate extraction approximates half the net chemical release. Of the lactate which appears in blood, most of this will be removed and combusted by oxidative (muscle) fibers in the active bed and the heart. The "shuttling" of oxidizable substrate in the form of lactate from areas of high glycogenolytic rate to areas of high cellular respiration through the interstitium and vasculature appears to represent an important means by which substrate is distributed, metabolic "waste" is removed, and the functions of various tissues are coordinated during exercise. During recovery from sustained exhausting exercise, most of the lactate accumulated during exercise will continue to be removed by direct oxidation. However, as the muscle respiratory rate declines in recovery, lactate becomes the preferred substrate for hepatic gluconeogenesis. Practically all of the newly formed liver glucose will be released into the circulation to serve as a precursor for cardiac and skeletal muscle glycogen repletion. Liver glycogen depots will not be restored, and muscle glycogen will not be completely restored until refeeding. This is because the diversion of lactate carbon to oxidation during exercise and recovery represents an irreversible loss of gluconeogenic precursor and because the processes of protein proteolysis and gluconeogenesis from amino acids are insufficient to achieve complete glycogen restitution after exhausting exercise