I do a lot of amusement park visits each year and have played with the sensitivity settings on my FitBit and phone apps enough to find the right combination so it counts steps but not rides or other non-step movement. I still sometimes get skewed "Active minute" counts and extra flights, but my step count seems to be right on. I'm always running two trackers between the phone and the FitBit Charge and they're usually within 100 steps of each other, and my friends that I'm with also end up with similar results between phones, wrist trackers and clip-on trackers, so I'm pretty confident in their accuracy.
That being said, my average day at most amusement parks is in the 15,000-20,000 range. Some are more and some are less, depending on how spread out the parks are - for instance, Cedar Point is consistently one of the highest, and Dollywood always results in an absurd number of stairs.
On Wednesday we did a "whirlwind" day at Disney - 3 parks in one day, trying to get the major stuff. Thanks to friends, we didn't have to wait for anything, and were able to get the major attractions in Epcot, DHS and MK, plus dinner at BOG. Total for the day was 30,000 steps. I'd say that's definitely on the high end because we were seriously rushing and covered a LOT of ground, but also probably pretty accurate. For my step length, which I have calibrated on my FitBit and is fairly short, that was around 13 miles. So, I'd say those who are doing a normal day, ending up with distances in the 8-10 mile range seems about right.