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

Or:

"Let's face it, 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".

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

Or:

"Let's face it, 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".