She's teasing him, of course, falling back into an old pattern of flirting that's too irresistible to avoid entirely. It's habit, and he seems to be as inclined to it as she is.

That means, of course, that what he's picturing is more or less exactly what she's thinking of.

By the time he's actually up to the challenge, she figures she'll have come up with some distracting ideas that don't involve her tits or her lips.

"Darling, at your fittest I'm not sure you'd be up to my worst. Though it would be fun to try."