Nonreligious version of "heaven knows that…"

Are there any nonreligious versions of the phrase "heaven knows"? For example:

Heaven knows I'm not perfect when it comes to exercise.

I've heard other religious variants like "the Lord knows", but I can't find a nonreligious version.

Goodness knows is a derivation of God knows, so it is still 'religious' but not overtly so.

goodness knows

1. No one knows. (A variant of "God knows.")
   Goodness knows how long it will take for my application to be processed.
2. It is true, certain, or definite that; it is obvious or clear that.

If you want to go aggressively secular, there is Fuck knows

fuck knows

(idiomatic, vulgar, followed by a wh-clause) I don't know; nobody knows; it is unclear. Fuck knows what we'll do now the car's broken down.

but while 'fuck knows' can substitute in many situations, the requirement for a subsequent wh-clause means it wouldn't work in your example construction.

Goodness knows... (when I'll see you again).

http://idioms.thefreedictionary.com/goodness+knows

OR,

Who knows/can tell...(when I'll see you again).

https://en.wiktionary.org/wiki/who_knows

E.g. "heaven knows I'm not perfect when it comes to exercise."

The real meaning of "heaven knows" in the given example is a statement of a fact that cannot be refuted (though it may not be immediately obvious). I may try to pretend I am good at it, but there is a higher being that knows the truth.

Therefore the secular equivalent would be something like:

"There's no denying I'm not perfect when it comes to exercise."