In some more subtle form, like in a conversation where you're talking about you being away/eating better/whatever, bring up her going to the gym with you and eating better now that you're back. Make it a partner activity, act as if you're planning on continuing like this and want her to join you in the new fun hobby you've picked up. Don't talk about it like her losing weight or for her to get into better shape/weight loss, but treat it more like couples time for you guys to have better habits and spend time together doing something positive and productive.

Personally I'd just be straight up though. I hate the sugar coated (pun intended) approach. Feels so disingenuous. She deserves to know the truth about how you view her (IMO).