Dr. Pepper Schwartz: I understand your situation, but I think there is hope if you get on an exercise and diet program together. You may not need to lose weight, but everyone can tone and shape into a better version of themselves. It will be good for you, too! Your wife is probably unhappy with how she looks and if you are there to support and guide her, it will make a world of difference.
Find out what kind of exercise she likes. Maybe start with one-mile walks, then build up to two miles, etc. Buy a pair of bikes and takes rides together exploring your town. Get a dog and walk it together. Join a coed dance class together. In other words, do fun activities that support the marriage, get her moving and get her feeling better about her body, and also into a regimen where she starts losing some weight and gains the belief that she really could look better.
As for nutrition, share meals instead of having them separately. Indulge in a splurge meal once a week, but help each other eat well every other time. Join Weight Watchers, Jenny Craig or a similar program together. If she is grossly overweight, ask her doctor what medical programs you can start.
Don't give up - be part of the solution. It is possible to get the wife you once desired back into a shape that will make you both happier and healthier.