You like walking. sign up to a local Meetup group for hiking. Start out slow. acquire some gear, and try an overnight. You'll burn tons of calories, meet some cool people and see some neat stuff!USPSA matches, Steel Challenge shoots, kayaking down the river, or walking around the lake behind the house.
Can't think of anything new outdoors that I've got on my list to try. Open to ideas.
All the best,
Andrew.