Standing hurts my feet within 10-20 minutes, but I found a treadmill/walking desk to be something I could do all day long. It helped a lot with back pain, and as a side-effect it cured post-lunch sleepiness.

Coding while walking was never a concentration problem for me, so I can only offer that as an anecdotal data point. It might be me, but maybe walking rather than standing makes it easier to concentrate?

There were plenty of downsides that make using the walking desk hard for me. I was working in games, and testing a 3d game while walking was often disorienting and would cause me to trip on myself or walk off the treadmill. Frequent visits and phone calls from other employees were more difficult to deal with than coding while walking. And a treadmill desk is bulky and noisy and hard to move around. I got the smallest setup I could find, and it still used up a lot more space than I expected.