Last year, I spent six days in the Kalahari, southeast Namibia, hard by the Botswana border. While generally flat, I spent six days walking in sand and up and down sand dunes, glassing and stalking. This was not walking on the beach in your barefeet, but up and down dunes that were probably 300-500' tall, carrying a rifle and pack - get to the top, crouch and look, leopard crawl and belly crawl. I'm in decent shape and I lost 8 pounds during the hunt. I'd bet an expensive lunch any outfitter in Namibia will put together a hunt where you will walk your a$% off if that's what you want to do. I saw pics of hunting Zebra in the mountains - while not particularly high elevations, I saw lots of boulders and rock that would make any hunt there on foot, a real challenge.
You may have some mis-conceptions about spot and stalk hunting in Africa - I leopard crawled for 40 minutes to get inside of 200 yards to kill a Rowland Ward red hartebeest. I never leopard crawled for any animal I've killed in the Western US or Alaska.
Good luck whatever you decide.