The trick to using a standing desk for long periods is having a padded wood platform to stand on. Standing on concrete or carpet on top of concrete will kill your legs in less than an hour. I have a slightly raised plywood platform with padding on top. I can stand on it all day with no problem.
I guess I don't get the need of the standing desk. Can't you just take regular mini breaks and go walk around a bit? It doesn't even need to be outside or for a long length of time. It would be healthier and cheaper than a standing desk.