I agree with HIA. You need to tackle this with honesty.
I'm assuming you still find her attractive. To start a conversation with her, you need to stress how gorgeous she is and how much you love her. Then go onto how worried you are about her long term health. STRESS THIS. Suggest that simple exercise together would be great for bonding and you would love to spend that time with her. Next suggest walking. It is the least scary exercise for those who hate exercise, since everyone has to do it everyday anyway. 3 hour long walks a week would be enough to reverse some of the damage.
Oh and when discussing the health aspect, maybe consider going to a dr with her and having both your cholesterol levels checked. Even if you are fit, if you are eating badly cholesterol can kill you.
Good luck.
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.