One of the biggest keys seems to be reduced mitochondrial energy production in various cells/ tissues/ organs, induced by COVID-19. This can happen even with mild cases, alas. There are various ways to induce mitochondrial neobiogenesis, exercise being amongst the most accessible. Basically tell your body it needs more mitochondria by exercising.
why does this game have the most pain in the ass way of allocating stat points, just let me click an arrow or something
Due to a programmer error relating to the cap on daily exercise and how the game stored dates you could get locked out of the gyms for effectively the remainder of the play through if you were unlucky with hitting daily limits on exercise.
(In addition, using a specific savepoint unlocked through main quest progression would just completely break the basketball minigame. San Andreas was a weird game, even by '00s jank standards.)