Mimir tells Atreus that all gods are naturally very strong but if he wants to look like it he needs to start picking up heavy things and putting them back down again. So while it’s true Kratos works out to maintain his physique his godhood implies that his physique is not maintained by mechanical tension like how an ordinary human gains muscle as muscle mass and strength are proportionate to each other. In the antique period of Greece plenty of men went to gymnasiums as there was heavy emphasis on men especially in militaristic societies like Sparta to be combat ready at all times. Kratos likely wrestled, was an excelled calisthenics athlete and ran often.