You are absolutely right, those type of foods really make you feel tired, blah, and unmotivated. I can tell when I'm eating too much junk food(which is always from Halloween all the till Christmas.:/ If it's not in the house I have no temptation to eat it and this is just baked goods and a little chocolate. This year(as always) I'm trying not to eat the cookies we make or others bring over with our cookie exchange. I do think many of us women have a harder time with willpower of not eating chocolate/candy then men. My husband refuses to eat any junk.
Eating right and exercising truly makes a difference! Even if its exercising a few minutes a day. Eliminating sugar, fats, saturated fats, ect make a huge difference in the way we feel. I can only bike a few minutes a day now and it really helps my mood improve. I use to be a runner once I met my husband. I now use superfood supplements in my diet and I feel so much better.
Your wife can only change if she wants to. If she's refusing to take your advice and even denying walks with you, she's not going to budge. If you threaten her, she may try to exercise and eat healthy, but most likely it will not last in the long run.
You could try asking her to go to fun places that include a lot of walking like the zoo, hiking on trails, ect. I'm not sure if it would work though.
I started my healthy path(except when pregnant) when I met my husband. He's extremely athletic and runs marathons, triathlons and races in ironmans. He does all the shopping, so there isn't much junk in the house. We don't totally eliminate it because the kids need snacks for school. He didn't ask me to, but since he only eats healthy foods, I switched over myself. We now teach our children the importance of home cooked meals and fruit/veggies for snacks at home. My children have had fast food only a few times in their life unless its subway. They actually don't like it.
If your wife likes to cook ask her to make meals for you that are healthy. Tell her that you really would like them. Maybe that would motivate her to eat them as well. Allrecipies.com is one of my absolute favorite websites. It has very good recipes and there are people who make revisions in the comments to make it healthier or buy a good diet cookbook. Amazon.com has great ratings telling you which are the better cookbooks. I love making my husband happy and I do take his advice seriously.
When I was a teenager I worked at a fast food joint. I ate there daily, sometimes twice in a day if I worked a double shift. I absolutely loved the food. Once I quit eating it(I haven't had FF for over 12-13 year) I no longer can stand it. It smells gross too.
I really wish you luck. I understand your concerns and as we age eating healthy is so important to our health. It helps with cholesterol, diabetes, keeps a healthy heart, ect..