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That's amazing, congrats!
I very much agree the best exercise of any is just simply going outside for a few hours for a brisk walk. Its the best because any age group can participate, it's very low impact on the knees and body in general, it elevates your heart rate, gets your blood going, helps you clear your head, and gets you outside in the elements and into the sunlight (vitamin D).
I recommend trying to walk on the grass and dirt over concrete if you can help it, or at least mix it up. Same thing for running btw. Do you have any issues with your knees and feet from running on the sidewalk/road? I don't like running on concrete so I get heavy cardio elsewhere.
I wouldn't completely skip the gym for good though. Or at least not what the gym has to offer, mainly weight lifting and resistance training. Beyond losing weight if you feel you aren't healthy, it's also very important to build and maintain muscle, especially as you get older, for both men and women.
This can be accomplished with a simple kettle bell or pair of free weights/dumb bells in the house, along with doing push-ups, sit-ups, squats and other in home/in garage exercises. So yeah, you don't need a gym membership to be in good shape at all. You are right about that.