What about nothing at all? 

*"I'm not perfect when it comes to exercise."* 

Or if it's an admission of sorts:

*"[To be honest][1], I'm not perfect when it comes to exercise."*

Or:

*"[Let's face it][2], I'm not perfect when it comes to exercise."*

There's a lot of ways to express the idea here, not necessarily requiring a directly equivalent substitution for "heaven knows".


  [1]: http://english.stackexchange.com/questions/182709/why-do-people-say-to-be-honest
  [2]: http://dictionary.cambridge.org/us/dictionary/english/let-s-face-it