We're 68+ years old and exercise daily...in some fashion. We started doing this as a couple when we were assigned to Keesler AFB in Biloxi, MS (1991-1993) and I commanded the Air Force medical center there. I had lost a considerable amount of weight as I got more into senior positions, but here I'd be one of a very small group of unit commanders and really had to set an example.
Lynnette and I decided to meet for "non-lunch." We'd come to the running track that was near my hospital and not far from her Family Support Center, bring a sack lunch and do much more exercising than eating. It worked well for both of us; I got down to two pounds over my high school weight.
Since then we've exercised regularly; Lynnette goes to a Pilates class at our health club twice a week and attends a Strong Women, Strong Bones class twice a week (lifting relatively light weight and taking them slowly through a range of motion (ROM) pattern). She also does daily stretches and goes to the gym two to four times a week with me. I go to the gym at least six days a week and ride a recumbent bike, do a series of stretches and use six weight machines. I've gradually built up my ride so now I burn ~650 Kcal. and ride >20 miles in 65 minutes on days when I'm feeling up to it.
But, we're both sensible. My right total knee replacement (done nine years ago) limits how much weight I should use on several machines; my tendency toward developing tendinitis means I limit ROM on two other machines. If I've done a lot on the bike the day before and my legs are tired, I go slower and add minutes to reach my calorie and mile goals.
We also vary our exercise: we dance, walk stairs, snow shoe (more about this later) and ride street bikes.
I tell people who see me ride and comment I must be in great shape that it took me seven years before I got to this point. I think "couch potatoes" should start by walking, a relatively short distance and most people at my age should see their doc before attempting anything strenuous. I also think there are lots of ways to burn a few extra calories; parking a long ways out from the store and walking back and forth to your car is a simple one. One friend said he hid his TV remote so he had to get up to change channels or turn the set on and off.
I don't think exercising is the way to lose weight, but it's a great adjunct and is clearly important in keeping weight off once you've lost some.