For me, I cannot walk slow for my exercise and expect it to keep me healthy. Why? If I walk 2-3 mph, I do not even break into the "aerobic zone" heartrate. If I walk at about 3.8-4.2 for an hour, I will almost always break into aerobic near the end of my walk. I don't claim to be much of an athlete, but for me, getting the HR up to the aerobic zone now takes more effort than when I started running in 2008...I used to get huffed and puffed with a brisk walk.
So, I think it really depends on the person...what level of fitness you have, your resting heartrate (I check every morning along with BP - 100/60 and 45bpm this morning), your lung health and capacity, and what you want to get out of it.
Here is my non-qualified opinion, based on my own research for my own health purposes: If you are not an experienced or avid walker, I would start slower/less mileage. No more often than once per week, add 10% to your mileage (i.e., go from 2 miles to 2.2 miles) and try to do it within the same time limit as before. Keep tabs on your heartrate. You want to get it into your aerobic zone for about 30 minutes each time, if you are doing this for your health. Once you have mastered each level, move up to the next level. You can tell you have mastered the level by your HR. If you begin finding that it is taking you longer to get into the aerobic zone, you know it is time to go up a notch.
Hope this helps. As always, YMMV, and remember, I am not a certified coach, nor am I a doctor. The above is only my opinion based on my own research into my own and my family members' fitness and is not intended to be medical advice, and cannot be taken as such. If you have not talked to a doctor, health coach, or other medical professional, you should do so before beginning any new fitness program. Only they can provide a QUALIFIED opinion on your readiness to exercise at any given level.
Good luck!
R