Over perhaps 100 trips of 3 to 40 days each, over the past 30 years, we have consistently averaged 18 or 19 miles per day on trail. That's 9 to 10 hours of hiking, and an hour for lunch, and arrival in camp with a few hours of daylight left. On days when we need to travel 22-24 miles, I feel like pushing and it doesn't feel leisurely. When we walk 18 miles it feels relaxed. For me, there's a big difference between 18 and 22 miles, the extra 2 hours of hiking means I feel rushed through lunch, don't set up camp in time to take a sponge bath while the sun is still shining, etc.
When hiking off-trail distances are irrelevant, as it all depends on the terrain and obstacles. And trails with no maintained tread (say the northern 100 miles of Vermont's Long Trail, or the description earlier in this thread of trails in New Zealand) -- those are slower and variable.
I think of it this way --
A leisurely walking pace on a good trail is 2 or 2.25 mph. A fast pace is 2.5-3.5 mph. That's assuming a good trail tread, so you can just stride and not worry about footing.
A full but not long walking day is 9-10 hours of walking (start at sunrise, hour off for lunch, end 1-4 hours before sunset, depending on season).
So a full-duration leisurely-paced day is 18-20 miles (9-10 hours at 2mph).
Or a short-duration fast-paced day is 18-20 miles (6-7 hours at 3mph).
Either way, 18-20 miles is a very reasonable target for somebody who has reasonable cardio-vascular fitness and hikes enough that all of their leg muscles are used to the effort.
People who want to sleep late, cook breakfast, stop often for breaks, stop early, or any combination of those -- if they also want to walk at a leisurely pace -- then 20 miles per day will be impossible.
For Jim and me, our sweet spot is 18 mpd, taken in the all-day, leisurely-pace style. We start walking at sunrise, walk at about 2.25 mph, but have brief frequent stops for birds, flowers, vistas, such that our actual pace is 2 mph of walking. We stop an hour for lunch, and walk until 2-3 hours before sunset. For us, that a is leisurely, comfortable, no-stress, no-rush, no-pain day.