It is very difficult to engage in exercise when other health concerns are at play. If exercise is very difficult with your present state, focus on eating right. Most weight loss specialists agree that weight loss has more to do with diet (up to 95% diet) versus exercise. Take a look at this article for advice: http://www.wikihow.com/Develop-Healthy-Eating-Habits
“Being active as we age can play a role in cognitive function, and reduce the risk of disease such as dementia and Alzheimer's,” says Amanda Paluch, a postdoctoral researcher at the Northwestern University Feinberg School of Medicine. “Research has explored several mechanisms, finding that exercise can increase synaptic plasticity and strength of nerve impulses in the brain, and have a positive effect on the hippocampus.”
Exercise has long been correlated with a longer life, but it’s only recently started to become clear why this might be. Studies, like a new one in the journal Preventive Medicine which found that exercise is linked to longer caps at the ends of chromosomes, have helped flesh this out a bit more. These caps, called telomeres, naturally shorten as we age, with each cell division. People who live a long time have telomeres that are in better shape than those who don’t—but there’s a lot we can do to affect the rate at which they shorten over the years. The team behind the new study looked at data from CDC's National Health and Nutrition Examination Survey, and found that for people who exercised regularly, their telomeres were 140 base pairs longer on average than sedentary people's. Which correlates to being years “younger” than their sedentary peers.