We're told ... Katy and Russell knew the relationship was on the rocks a couple of months ago -- but didn't want to give up on the marriage without making a concerted effort to work things out.

But the effort fell flat ... with both sides feeling "it just wasn't there." We're told both Katy and Russell were on board with the divorce a few weeks ago -- but they didn't want to be in town when the papers were filed ... so he went back to England and she booked it to Hawaii.

Since Katy's parents are evangelical Christians, we're told she didn't want to be the one to "officially" end the marriage by filing the docs ... since she was raised to believe divorce is wrong.