The biggest issue for me is foot fatigue. I now use 2 anti-fatigue mats - a rubber lower mat (Genuine Joe Anti Fatigue Mat), and a foam upper (Imprint Cumulus9 Kitchen Mat Nantucket Series). Without these there is no way I could handle more than 6 hours. However, even with them, I still have sore feet after 10-12 hours (8 hours is ok). Another useful component of a standing desk is a foot stool to periodically shift weight around.
Desk Pics: https://photos.app.goo.gl/rQBWwBQx97liILOn1 https://photos.app.goo.gl/CVrK8p1ukau5kLiw2