I'm very impressed with your early morning walking routine! Good for you!

I walked 4 miles per day for years, and often added a longer 10 mile hike once a week to that. It wasn't too much for me, even at the higher end of my weight range.

Walking is such a natural, safe form of exercise, all you need to do is listen to your body. If your knees or ankles start to hurt, take a day off. Otherwise, keep up the great habit!