I was in the same position as you. I was an avid sportsman during school. But once I started university, everything started going down hill. Fortunately, I realized early that this is not the way I want to live and started doing sports again. But as responsibility grows, there is less and less time to do 3 hours of your favorite activity, so I started walking.

I still do go to the gym but walking is somewhat my meditation. Congrats on losing 50+ lbs, keep pushing and going. The secret(or what I like to call common sense) is consistency which in turn becomes habit. Keep on walking for the next 3 years or so and try stop for a week. You will actually miss walking because your body is used to it now.