I don't recommend trying to wrap it in a lie like "We need to be healthy so we live longer," she's probably seeing right through that. You're also giving her an escape route e.g. "I'm already at a healthy weight."
I think you're going to have more success just being supportive, filling up the fridge with healthy food, doing exercise as a couple. Reward her for good work, use your imagination. Carrot works better than stick.
Last thing you want to do is be confrontational or negative, a lot of women associate their self worth with their appearance, even banal critiques are going to cut deep, shit will blow up.
That being said, if she doesn't want to change, it won't happen.
