At my office, I used a fixed-height standing desk without the bar-stool height chairs that are commonly seen paired with fixed standing desks. In other words, I am standing for the full work day. Meanwhile, my desk at home is a traditional sitting-height fixture. So I do a fair amount of sitting in the evenings as well.
For what it's worth, I don't perceive any difference in my ability to concentrate or be effective while standing, with some caveats:
* With the unfortunate advent of glossy-screened displays, there can be additional glare at a standing height. You may want to evaluate whether your concentration problem is related to the amount of glare or outright reflections of nearby activity you are perceiving on your display. For me, it was important to get semi-matte displays (truly matte is ever harder to find these days) and orient the displays to minimize glare. Side rant: hardware manufacturers, please make matte displays mainstream again.
* My desk was quickly hand-built out of plywood and is non-adjustable. It was intended to be experimental but I've used it for about two years now. My intent is to replace it with a hardwood fixture soon. If you aren't interested in splurging for an adjustable desk, don't make a stupid mistake I made: I forgot to factor in the height of the keyboard itself (approximately 1.5 inches for my mechanical keyboard) when planning the height of the desk. I measured the distance away from the floor that I wanted my hands to be at, but forgot that in practice they would be 1.5 inches higher. This is a continuous annoyance making me look forward to replacing this "temporary" desk.
* As others have said, a gel pad may be helpful. I think this may be a matter of taste. I personally use one, but I sometimes move it away since it can cause a weird "floating" sensation at times.
* I do a lot of periodic movement while I am working or thinking. I squat, leg-lift, walk, sway, stand on one foot, etc. I feel this helps keep me from feeling "locked up" in a standing position.
* If you can situate your desk so that you are back up against a wall, you can also do a lean-back arrangement which is pretty comfortable.