In my experiences and humble opinion, exercise is critical for long-term weight maintenance and the latest governmental guidelines for weight loss/maintenance agree - check out the quote in my signature.
94% of maintainers in the National Weight Control Registry exercise for an average of an hour per day and the #1 exercise choice is walking. So to answer one of your questions, for many maintainers, walking is serious enough exercise.
Why is exercise so essential? First, exercise is necessary for good health and we're doing this for health, right? Your heart (and everyone else's) needs a minimum of 30 minutes of cardiovascular exercise three times per week (that's exercise that raises and sustains your heart rate to at least 70% of maximum). And that's just for general good health, not weight loss or maintenance.
Second, it's difficult to keep your calories low enough on a daily basis to keep weight off without the additional calorie burn from exercise. To go back to the NWCR, the average member is maintaining on 1400 daily calories (and remember, that's with the average hour of exercise). Without the calories burned from exercise, that 1400 number would have to be lower and that's tough to do day-in and day-out. Don't be mislead by metabolic calculators showing that you 'should' be able to maintain on 2000+ calories - it's not true for many of us, especially those of us trying to maintain large losses.
Third, when you lose weight without exercising to sustain your muscle mass, up to 40% of what you lose is muscle, not fat. The result can be a normal BMI but an unhealthy body composition (ratio of body fat to muscle mass). Because muscle is the calorie burner in your body (every kg of lean body mass burns 50 calories/day), the loss of muscle will lower your metabolism and make it harder to maintain your weight loss because you'l have to eat so much less. Exercising with weights will maintain and build your muscle mass, make you smaller and tighter, help prevent osteoporosis, and keep your metabolism running high.
Finally - and this one is kind of intangible - I think that exercising keeps you in touch with your body. Many of us became overweight/obese through denial and completely losing touch with how our bodies look and feel. Exercising forces you to use and experience your body. There's a world of lessons about perservance, meeting goals, overcoming obstacles, and inner strength in exercise and they all apply to weight loss/maintenance, goofy though that may sound.
In the end, we all have to do what works for us and obviously what you're doing has worked well for you so far.
If 94% of maintainers are exercising, then 6% aren't and maybe that's the group you're comfortable with.
But if you want to try to incorporate some exercise, walking is a great place to start. But ... please consider the addition of some weight training also, even if it's just working out with some light dumbbells or resistance bands at home.
Exercise?
)...
