what does "she's a bit of a fitness fanatic" mean? Does it mean that she treats it too seriously? I know that after 'a bit of' you add a negative adjective, so I presume that the meaning should be slightly negative.
It's not necessarily negative. It means she loves physical fitness (excercising and maintaining a healthy diet) as much as some people love watching football or Star Wars.
"Susan is a bit of a fitness freak. She runs five miles a day, every day, no matter what the weather."