In a way your stamina does increase as the game progresses. Skills taking "less stamina" over time is really just your muscles growing to make that work easier. The only reason your base stamina is so low is either because you recently died, rather hard on the body I'd imagine, or else you're starving. And while a marathoner may have a ton of stamina, if they were starving they wouldn't exactly have as much stamina as they otherwise would but they could still probably run longer than a well fed couch potato.