@kingjamesversionbibleonly Most of the changes I notice when adding daily walking were increased hunger and ability to maintain my weight. I didn't lose any weight but I also haven't been trying to. For context, the walking I've been doing is at least 5-10km on flat ground per day, and up to 17km (though this happens less often), with a weighted backpack. It was really tiring at first but I can do it no problem now.
A few years back I was being paid to hike (as a geologist and again with a very heavy backpack) and was walking anywhere between 5k-20k per day for about 5 months and had the same experience: I ate more, my cardio was a little better, and I was maintaining my weight but my clothes were getting baggier so presumably my muscle:fat ratios were changing for the better. The ground was flat to very steep. I didn't make any effort to track or restrict calories.
But all the way back in 2005, I was yet again doing a lot of hiking (as a geologist) in a very hilly to mountainous area. I was also restricting my calories somewhat and lost a lot of weight over the course of 6 weeks. The hills/mountains - usually summitting at least one per day - meant my cardio was strong.
So this is my roundabout way of saying walking is great as a low impact exercise, great for my moods, and good for maintaining weight if you're not restricting calories. If you're restricting, it's an easy way to lose some weight. If you can add hills for some extra cardio, even better. I love walking and hiking!
To answer your other question, how far you'd have to walk to notice any difference depends on what you're doing now, both with exercise and eating. You'd be best placed to figure it out by going for a walk. Maybe try 2km and see how you feel and build from there.
no noticable difference in my composition. I live in a very flat area so it's not giving my heart and lungs a good workout. My feet and ankles are still sore by the end (cheap shoes). Better than nothing.