If done properly, walking is the complete exercise that will keep you fit for life. Most people view walking as "strolling around" with your hands in your pockets. It can be that, of course. But there is so much more to walking that that.
There is "brisk walking" in which one walks full-speed ahead up to a speed of 15 minutes per mile. This is the kind of walking I did for the first three months of my program. I eventually got 4 miles walked in an hour.
After brisk walking, you can then move up to "aerobic walking" which is when you put your arms at roughly 90 degree angles, allowing your legs to move much faster. (When your arms are hanging down at your sides, you are limited in how fast you can walk). I have recently started walking that way and can now do 14-minute miles. Thus, I am starting to pass slower joggers by walking! Using this gait, one can get down to 12-minute miles or even less. But it does take a lot of work.
Finally, there is something called "race-walking" which is just an insane kind of walking. Here, you can walk as fast as most people run. To get to this level, you are an athlete of the highest order.
I don't think I'll ever be a race-walker but I can certainly get down to 12-minute miles in the near future by walking aerobically.
So why not just jog? Well when you jog (or run), you are putting three times your body weight on your feet everytime they touch the ground. This is a real pounding that will catch up to you sooner or later with injury. Even elite runners get injured eventually. On the other hand, walking is an exercise you can do for life with a very low risk of injury.
Hiking in the mountains is also good if you do it fast enough to make you breathe hard.