You'd be very lucky to find the "right" saddle the first time. Unfortunately, for many if not most, it does take many hundreds, if not thousands of miles to find the right fit (or to break something in).
An analogy for long distance hikers is shoe fit. There's no way to tell how a shoe is going to work for many consecutive twenty-plus mile days by getting measured or trying it out in the store, or even on five-mile walks. At the beginning of my hiking career, it took me over 2000 somewhat painful miles (that's millions of steps) to find the right shoe, and every one I tried was nominally the same size.
I'm still looking for the right saddle. For instance, the one I'm using now doesn't feel perfect in the crotch, but interestingly a nagging neck pain issue has improved. So maybe I'll keep it for a while.