Of course you're going to get fatigued if you're not used to standing for long periods of time. You're constantly engaging muscles that you let lie comfortably for most of the day when you get into the office and sit or slouch into your chair.

Of course it's going to take training, as building up any muscles does, and it's going to be tough while you're doing it.

I'm glad to see others here saying they're happy with their standing desks. It's something I've been really eager to try but haven't been able to for a number of reasons yet.

Whatever the future is, I don't think it's sitting at 90 degrees at a small desk for long periods of every day.

Personally I find it very hard to recline and get work done. I find it harder to engage with what I'm doing, get drowsy more quickly, etc.