how on earth are you supposed to up your fitness skill? i've never seen any exercise prompts. someone please help me!

it'd be nice in general if the game told you how to increase/decrease skills (like what effects them) or when skill checks happen. and how the nighttime increase skill works. the game says "Every 5 times you pass a skill check you get a chance to increase a skill when you sleep at night. Likewise, every 5 times you increase a skill, you get to increase a stat when you sleep"  but every night i get the option to increase any one of the skills i choose?

I see the confusion. You don't have to use a specific skill to increase it. Any 5 times you succeed at a skill check, regardless of which skills you used, you get to increase any one skill of your choice. 

I chose not to make skill checks obvious to make things more immersive. I think it was both a pro and a con. I haven't decided if I'll do that in the sequel name or not.